Take Gifts From the Richest Pile
10 Prerequisite LeetCode Problems
For the Take Gifts From The Richest Pile problem, the following problems are good preparation:
Kth Largest Element in an Array: This problem helps you understand how to handle ‘k’ in relation to an array, which is important for our problem.
Merge k Sorted Lists: This problem helps in understanding priority queue usage which is critical for solving our problem.
Top K Frequent Elements: Similar to our problem, it focuses on the ‘k’ most elements, which in our case are the richest gift piles.
Squares of a Sorted Array: This problem can help in understanding the manipulation of array elements, a concept used in our problem.
Heaters: Working with this problem can help understand the idea of distributing resources (in this case, heat) in an efficient way.
Sliding Window Maximum: This problem helps in understanding how to handle and update maximum values in a set of elements.
Last Stone Weight: It involves a similar concept of reducing the largest values until we reach a final result.
Find K Pairs with Smallest Sums: This problem helps understand priority queue usage and dealing with ‘k’ pairs, a concept similar to picking ‘k’ piles in our problem.
Third Maximum Number: This problem deals with finding the ‘k’ maximum number in an array, a concept that’s related to our problem.
Array Partition I: It helps understand how to break down and manipulate arrays for maximum result, a skill needed for solving our problem.
Problem Classification
The problem can be categorized into the “Mathematics and Simulation” domain.
What Components:
 An array of integers representing the number of gifts in various piles.
 A number ‘k’ denoting the number of seconds or turns.
 The operation that is performed every second:
 Pick the pile with the maximum gifts.
 If there are multiple piles with the maximum gifts, choose any pile.
 Leave behind the square root (rounded down) of the number of gifts in the pile and take the rest.
 A requirement to return the number of gifts remaining after ‘k’ seconds.
The involves interacting with an array and performing a mathematical operation repeatedly until a certain condition (k seconds) is met. It could be further classified as a “Simulation” problem, as we simulate the process of picking up gifts for ‘k’ seconds, while also involving “Greedy” aspects, as in each turn, we’re always picking the pile with the maximum gifts.
This requires knowledge of array manipulation, mathematical operations (like square root and floor), and a way to track the pile with the maximum gifts efficiently.
Clarification Questions
Can we assume that the input array ‘gifts’ will always have at least one element, and each element will be a positive integer?
How should we handle the case when there are multiple piles with the maximum number of gifts? The problem statement says we can choose any, but do we need to follow any specific strategy like choosing the first one we encounter?
What should be the output if the number of seconds ‘k’ is larger than the number of gifts? Do we return 0 or is there a special handling required?
The problem mentions leaving behind the floor of the square root of the number of gifts. Does this mean that, if the number of gifts is less than 2 (since the square root of any number less than 2 is less than 1 and the floor value of any decimal less than 1 is 0), we take all gifts from the pile?
Are we allowed to use external libraries for priority queue operations and mathematical functions, or do we need to implement everything from scratch?
Problem Analysis and Key Insights
Priority Queue: The problem involves selecting a pile with the maximum number of gifts in every operation. This is a typical scenario where a data structure like a max heap or priority queue can be beneficial. The heap can store the piles, and we can extract the pile with the most gifts in O(log n) time.
Mathematical Operation: The operation of leaving behind the floor of the square root of the number of gifts in the pile indicates a mathematical computation on the elements. This could be computed efficiently using a standard math library.
Time Constraint: The problem mentions a specific time limit ‘k’ within which the operations are performed. We need to ensure our solution respects this constraint.
Multiple Steps: The problem involves a sequence of steps  selecting the pile, performing the operation, and repeating the process. This suggests an iterative approach would be beneficial.
Final State: The problem is not asking for the sequence of operations or the intermediate states of the piles but just the final count of gifts remaining. So, we don’t need to maintain the entire operation history.
Nonunique Maximums: In case of multiple piles with the same maximum number of gifts, any one of them can be selected. This gives us some flexibility in our approach and removes the need for unique prioritization.
Problem Boundary
The scope of this problem involves understanding and applying concepts from the following areas:
Data Structures: The problem entails managing a dynamic collection of items (the piles of gifts) where the item with the highest value needs to be efficiently retrieved and updated. This suggests the use of a data structure like a priority queue or a maxheap.
Mathematical Computations: The problem requires performing a mathematical operation on the number of gifts, i.e., taking the floor of the square root of the number of gifts.
Iteration and Looping: Since the action needs to be performed for ‘k’ seconds, the solution would involve looping or iterating for ‘k’ times, making sure to account for the possibility of ‘k’ being larger than the number of piles.
Problem Analysis and Understanding Constraints: The problem involves understanding and working within given constraints  like the number of piles, the number of gifts in a pile, and the time limit ‘k’. Understanding these limitations is key to designing an effective solution.
The problem does not involve deeper aspects like recursive problem solving, complex graph theories, or advanced data structures, keeping the scope confined to the above areas.
Establishing the boundary of a problem involves understanding its constraints and the limitations within which the solution should operate. For this problem, the boundaries can be determined as follows:
Input Size: The number of piles (gifts.length) is between 1 and 103. Each pile can have up to 109 gifts. The time for which you can pick gifts, denoted by ‘k’, is between 1 and 103. The solution should be efficient enough to handle these input sizes.
Operation: Every second, you pick the pile with the maximum gifts, take all gifts except for the floor of the square root of the number of gifts in that pile, and update the number of gifts in the pile. This operation repeats ‘k’ times.
Output: The solution should return the total number of gifts remaining after ‘k’ seconds.
Constraints: You can only pick gifts from one pile per second, and the pile must be the one with the most gifts. If there are multiple piles with the same number of gifts, you can choose any. Once a pile is chosen, the remaining gifts are the floor of the square root of the current number of gifts in the pile.
Understanding these boundaries will help to devise a solution that works within these constraints and handles edge cases effectively. It also aids in designing appropriate data structures and algorithms for the problem.
Distilling the Problem to Its Core Elements
The fundamental concept this problem is based upon is priority queue management and simple arithmetic operations. Priority queue is a data structure that keeps the elements in a certain order, with the highest priority element at the front. In this case, we’re interested in the pile with the maximum number of gifts, so we could use a maxpriority queue.
To describe this problem to someone unfamiliar with the subject, imagine having several heaps of candies. Each second, you select the heap with the most candies. Then, you take away most candies from that heap, only leaving a few (specifically, the whole number closest to but not larger than the square root of the original number of candies). After doing this for a certain number of seconds, you want to know how many candies are left in total.
The core problem we’re trying to solve is determining the total number of gifts remaining after ‘k’ seconds, given the rules of choosing and removing gifts from the piles. We want to simplify the problem by breaking it down into parts:
 Identifying the pile with the maximum number of gifts.
 Calculating the square root of the number of gifts in the pile, taking the floor of this value, and updating the pile.
 Repeating steps 1 and 2 for a specified number of times (seconds).
 Summing up the remaining gifts in all the piles.
The minimal set of operations we need to perform to solve this problem includes:
 Priority queue manipulation: initialization, insertion, extraction of maximum.
 Arithmetic operations: square root, floor, subtraction, and addition.
Visual Model of the Problem
A good way to visualize this problem would be to imagine the piles of gifts as an array where each element represents the number of gifts in each pile. A timeline can be used to represent the passing of seconds, with operations being performed on the array at each second.
Let’s take the example from the problem statement:
Initial State:
Piles of Gifts: [25,64,9,4,100]
Time (k): 4
After 1st second (Choosing the 5th pile with 100 gifts):
Piles of Gifts: [25,64,9,4,10]
Time (k): 3
After 2nd second (Choosing the 2nd pile with 64 gifts):
Piles of Gifts: [25,8,9,4,10]
Time (k): 2
After 3rd second (Choosing the 1st pile with 25 gifts):
Piles of Gifts: [5,8,9,4,10]
Time (k): 1
After 4th second (Choosing the 5th pile with 10 gifts):
Piles of Gifts: [5,8,9,4,3]
Time (k): 0
Total gifts remaining after 4 seconds: 29
This visualisation helps in understanding the progress of time and the changing state of the gifts pile array. It clearly shows which pile was chosen at each step and how the number of gifts in that pile was reduced.
Problem Restatement
Here’s a paraphrased version of the problem:
We have a list of integers, each integer representing the quantity of gifts in a pile. Every second, we perform the following operations:
 Identify the pile containing the maximum number of gifts.
 If there’s more than one such pile, we can choose any.
 Calculate the square root of the number of gifts in that pile, round it down to the nearest whole number, and leave this many gifts in the pile. The rest of the gifts from that pile are removed.
 Repeat this process for a specified number of seconds,
k
.
Our goal is to find out how many gifts would be left in total across all piles after k
seconds have passed. We need to return this total number.
The problem imposes a few constraints:
 The length of the
gifts
list is between 1 and 103.  Each integer in the
gifts
list is between 1 and 109.  The value of
k
is between 1 and 103.
The main challenge here is to efficiently perform these operations, keeping track of the maximum pile, and updating it every second for k
seconds.
Abstract Representation of the Problem
Here’s an abstract representation of the problem:
We have a list of numbers, each representing a resource quantity. In each iteration (up to a given number of iterations), we perform the following steps:
 Identify the highest number in the list.
 If there is more than one such number, choose any.
 Reduce the selected number by a rulebased operation, which is keeping the floor of its square root and removing the rest.
 Repeat steps 1 to 3 for the given number of iterations.
Finally, our task is to return the total sum of the remaining quantities in the list after all the iterations.
This abstract representation removes the specifics of ‘gifts’, ‘piles’, or ‘seconds’, and focuses purely on the operations and the structure of the problem.
Terminology
Here are a few key terms and concepts that play an important role in this problem:
Array: This is a fundamental data structure in programming. It allows us to store multiple values of the same type in a single variable. In the context of this problem, we’re using an array to store the number of gifts in each pile.
Floor function: In mathematics, the floor function is the function that takes as input a real number and returns the largest integer less than or equal to the input. In Python, it is implemented as
math.floor()
. In this problem, we use the floor function to calculate the number of gifts left in a pile after choosing it.Square root: The square root of a number is a value that, when multiplied by itself, gives the original number. In Python, it’s implemented as
math.sqrt()
. In this problem, we use the square root to calculate the number of gifts to leave behind in a pile after choosing it.Max function: A function to find the maximum value in a collection. In this problem, it is used to find the pile with the maximum number of gifts.
These concepts are crucial for this problem as they are used directly in the operations needed to find the solution. Understanding them will make it easier to grasp the problem and develop a solution for it.
Problem Simplification and Explanation
Sure, let’s simplify this problem and explain it through an analogy.
Imagine you’re at a buffet party with different food stations (gift piles). Each station has a different number of dishes (gifts). Now, you’ve decided to try dishes from the station that has the maximum number of dishes. However, you can only carry so many dishes in one go, and your carrying capacity is determined by the square root of the number of dishes at that station. To simulate this, you leave behind the floor value of the square root of the number of dishes at the station. You keep doing this until you run out of time (k seconds).
To give a concrete example, if one of the stations has 25 dishes, you would first leave behind floor(sqrt(25)) = 5 dishes. So, you would take 255=20 dishes. If you returned to the same station next, it would have 5 dishes remaining, so you would leave behind floor(sqrt(5)) = 2 dishes and take 52=3 dishes, leaving 2 behind.
This process continues, with you always choosing the station with the most dishes and taking as many dishes as you can (the total number of dishes at the station minus the floor of the square root of the number of dishes at the station).
The key concepts involved here are:
 Array/Data structure: The buffet stations and their dishes are analogous to the array of gift piles and their gifts.
 Choosing the Maximum: You’re always choosing the station with the most dishes, similar to how you need to always select the pile with the maximum number of gifts in the problem.
 Square Root and Floor Function: The rule that determines how many dishes you leave behind is the same as the rule that determines how many gifts are left in a pile after choosing it.
 Iteration: Just like how you repeat the process of choosing a station and taking dishes for a certain amount of time, the problem requires iterating through the process for a given number of seconds (k).
At the end of the party (or after k seconds), you count the total number of dishes left in all the stations. That’s analogous to returning the total number of gifts remaining after k seconds in the problem.
Constraints
Analyzing the problem statement and constraints, a few specific characteristics stand out:
Maximization: At each step, we’re choosing the pile with the maximum number of gifts. This makes it likely that we’ll need a data structure that can efficiently retrieve and update the maximum element, such as a max heap.
Square Root Function: We’re leaving behind the floor of the square root of the number of gifts in the chosen pile. Since we’re dealing with integers, and the square root function increases slower as the input gets larger, we can expect the number of gifts we leave behind to increase very slowly. This means piles with a large number of gifts will quickly get depleted.
Numerical Ranges: The number of gifts in a pile can go up to 10^9. This large range suggests that a linear scan of the gift piles at each step would be inefficient. Again, a data structure like a heap that can quickly retrieve the maximum would be beneficial.
Fixed Number of Operations: We’re performing a fixed number of operations (k), which could be smaller than the number of piles. This suggests that the problem might not necessarily require us to process every pile.
Order doesn’t matter: The problem doesn’t specify any order in which we need to process the piles. This gives us flexibility in choosing the order that is most efficient for our algorithm.
In summary, the problem appears to revolve around efficiently selecting and updating the maximum pile over a fixed number of steps, which suggests the use of a heap or similar data structure. The nature of the square root function and the large range of possible gift counts further reinforce the need for an efficient approach to handling the piles.
Looking at the constraints, several insights emerge that can guide our approach to solving this problem:
Gift Count: The fact that a pile can have up to 10^9 gifts is a significant constraint. It indicates that our solution must be able to handle large numbers efficiently.
Number of Piles: There can be up to 10^3 piles. This is a moderate number and indicates that, if necessary, iterating over all piles could be part of an efficient solution.
Number of Operations: We perform exactly k operations, where k can also be up to 10^3. This means we won’t necessarily interact with every pile, further suggesting that we need an efficient way to choose which piles to interact with.
Unique piles: Since every pile has a unique number of gifts (as inferred from the problem statement), there’s no need to worry about how to break ties when selecting the largest pile.
Given these constraints, an approach that can efficiently select and update the largest pile in each of k operations seems necessary. A priority queue, or heap data structure, which can maintain the max element at the top and adjust as elements are added or removed, is wellsuited to this task.
Moreover, because of the square root operation, the number of gifts in a pile decreases rapidly after a few operations. This suggests that it might be beneficial to spread the operations across different piles instead of depleting one pile at a time.
These insights can help guide the formulation of a potential solution strategy.
Case Analysis
Sure, let’s consider some additional examples:
Case 1: Single pile with minimum gifts (Edge Case)
Input: gifts = [1], k = 1
Output: 1
Reasoning: There's only one pile with the minimum number of gifts. No matter how many seconds pass, you won't be able to remove any gifts from this pile because the floor of the square root of 1 is 1.
Case 2: Single pile with maximum gifts (Edge Case)
Input: gifts = [1000000000], k = 1
Output: 31623
Reasoning: There's only one pile with the maximum number of gifts. After one second, you will take away the floor of the square root of the number of gifts, leaving behind 31623 gifts (because the floor of the square root of 10^9 is approximately 31623).
Case 3: All piles have the minimum number of gifts (Boundary Case)
Input: gifts = [1,1,1,1], k = 4
Output: 4
Reasoning: All piles have the minimum number of gifts. Even after k seconds, you won't be able to remove any gifts from these piles. So, the total gifts remaining are 4.
Case 4: All piles have the maximum number of gifts (Boundary Case)
Input: gifts = [1000000000, 1000000000, 1000000000, 1000000000], k = 4
Output: 126492
Reasoning: All piles have the maximum number of gifts. After each second, you will take away the floor of the square root of the number of gifts from a pile, leaving behind 31623 gifts in each. So, the total gifts remaining are 31623*4 = 126492.
Case 5: Time exceeds the number of piles (Typical Case)
Input: gifts = [25,64,9,4,100], k = 10
Output: 4
Reasoning: The number of seconds is greater than the number of piles. After 5 seconds, all piles would have gone through the square root operation at least once, leaving [5,8,3,2,3]. For the next 5 seconds, the largest pile is chosen each time, which is the first one. After 5 square root operations, it has 1 gift left. So, the total gifts remaining are [1,8,3,2,3] = 17.
These cases cover a variety of situations including minimum and maximum values, equal number of gifts in all piles, and more time than piles.
Visualizing this problem can be done with simple diagrams, where each pile of gifts can be represented by a box or a column in a bar chart. Here’s a textual representation:
Case 1: Single pile with minimum gifts (Edge Case)
Before:
Pile 1: 1
After 1 second:
Pile 1: 1
Case 2: Single pile with maximum gifts (Edge Case)
Before:
Pile 1: 1000000000
After 1 second:
Pile 1: 31623
Case 3: All piles have the minimum number of gifts (Boundary Case)
Before:
Pile 1: 1 Pile 2: 1 Pile 3: 1 Pile 4: 1
After 4 seconds:
Pile 1: 1 Pile 2: 1 Pile 3: 1 Pile 4: 1
Case 4: All piles have the maximum number of gifts (Boundary Case)
Before:
Pile 1: 1000000000 Pile 2: 1000000000 Pile 3: 1000000000 Pile 4: 1000000000
After 4 seconds:
Pile 1: 31623 Pile 2: 31623 Pile 3: 31623 Pile 4: 31623
Case 5: Time exceeds the number of piles (Typical Case)
Before:
Pile 1: 25 Pile 2: 64 Pile 3: 9 Pile 4: 4 Pile 5: 100
After 10 seconds:
Pile 1: 1 Pile 2: 8 Pile 3: 3 Pile 4: 2 Pile 5: 3
Each “” symbol represents a pile of gifts, with the number inside denoting the number of gifts in that pile. After the specified number of seconds, you can see how the number of gifts in each pile decreases.
Upon analyzing these cases, we can gather some key insights:
Nature of the problem: The problem involves choosing the pile with the most gifts, taking all but the floor of the square root of the number of gifts in that pile, and repeating this process a certain number of times. This involves keeping track of the maximum in a collection and modifying it, which implies a potential use of data structures like heaps for efficiency.
Impact of the floor function: The use of the floor function in the problem means that for piles with 1 or 2 gifts, no gifts can be taken, which effectively becomes an invariant in our problem. This can be seen in cases where all piles have a minimum number of gifts.
Role of
k
: The parameterk
determines the number of times we can take gifts from the piles. Ifk
is larger than the number of piles, we might end up taking gifts from the same pile multiple times, which could influence the strategy of which pile to take gifts from first.Impact of initial gift distribution: The initial distribution of gifts in the piles can drastically affect the number of remaining gifts. If all piles initially have the same number of gifts, the remaining gifts after
k
seconds will be equal irrespective of the order in which the piles are chosen.Edge cases behavior: In edge cases where there’s only one pile or all piles have the minimum/maximum gifts, the problem becomes more predictable. For a single pile, the pile is depleted down to the floor of its square root every second. For multiple piles with the same number of gifts, each pile behaves independently.
Behavior over time: With each passing second, the maximum number of gifts in any pile will decrease, often dramatically. This can cause the problem to become “easier” over time, as the difference between the pile sizes decreases.
These insights help us to understand the dynamics of the problem better, which will aid us in formulating a strategy for solving it.
Identification of Applicable Theoretical Concepts
This problem has elements that relate to various algorithmic concepts and mathematical properties. Let’s break it down:
Greedy Algorithm: The problem involves making a decision at each step that appears to be the best at that moment. In this case, choosing the pile with the maximum number of gifts at each second is a greedy decision.
Priority Queue / Heap: This problem requires us to find and remove the maximum element from a collection multiple times. In computer science, a data structure known as a “priority queue” or a “heap” is particularly good at performing these operations efficiently. In Python, we can use the
heapq
module which provides an implementation of heaps in the form of a minheap. To use it as a maxheap, we can store the negation of the values.Mathematical Square Root Property: The problem requires us to repeatedly take the floor of the square root of a number. This operation has the property of reducing numbers rapidly at first, then slowly as they get smaller.
Iterative Process: The problem involves an iterative process that is performed
k
times. Understanding how iterative processes work is key to solving this problem.
By identifying these mathematical and algorithmic concepts, we can start to formulate a strategy for solving this problem efficiently. The insights derived from these concepts can guide us in making design choices in our algorithm.
Simple Explanation
Sure, let’s think of this problem in terms of a child’s game.
Imagine you’re playing a game where you have different piles of toys. Each pile has a different number of toys. Now, every turn (or second), you can choose one pile and take most of the toys from it. But there’s a rule: you have to leave behind a certain number of toys. The number you leave behind depends on the current size of the pile. You have to leave behind as many toys as the length of one side of a square that has area equal to the current number of toys. And you take all the rest.
Now, let’s say you have 4 piles of toys, with 25, 64, 9, and 4 toys respectively. On the first turn, you can take most of the toys from the 64toy pile, but you have to leave behind 8 (since a square with an area of 64 has a side length of 8). So, you take 56 toys, and 8 remain in the pile.
You keep taking turns and choosing the pile with the most toys until you’ve taken a specified number of turns. After those turns, some toys will remain in the piles. The game asks you to count how many toys are left.
So, this problem is basically about playing this game optimally and figuring out how many toys are left after a certain number of turns.
Problem Breakdown and Solution Methodology
Sure, let’s go step by step. We’ll take the example from the problem statement: gifts = [25,64,9,4,100]
and k = 4
.
Start by identifying the pile with the maximum number of gifts: In this case, we start with the pile of 100 gifts.
Determine the number of gifts to leave behind: The rule is to leave behind the number of gifts equal to the floor of the square root of the current number of gifts. In this case, the square root of 100 is 10, so we leave 10 gifts behind.
Remove the taken gifts from the pile: After we’ve taken the gifts, we need to update the pile’s count. We subtract the number of gifts taken from the pile’s total, which results in the updated piles:
[25,64,9,4,10]
.Repeat this process for k turns: We continue with these steps for 4 turns.
The approach will look something like this:
 Turn 1: Select pile of 100 gifts. Leave 10 gifts behind. Updated piles:
[25,64,9,4,10]
.  Turn 2: Select pile of 64 gifts. Leave 8 gifts behind. Updated piles:
[25,8,9,4,10]
.  Turn 3: Select pile of 25 gifts. Leave 5 gifts behind. Updated piles:
[5,8,9,4,10]
.  Turn 4: Select pile of 10 gifts. Leave 3 gifts behind. Updated piles:
[5,8,9,4,3]
.
After 4 turns, we’ve left with 29 gifts in total.
To think of this problem in everyday terms, imagine you’re taking apples from baskets. Each turn, you take most of the apples from the fullest basket but always leave behind a certain number of apples, which is determined by the square root rule.
As you can see, the pile of gifts with the maximum number will always decrease significantly after each operation, which implies that we are basically working on the maximum element for each operation. This also suggests that using a data structure like a maxheap, which can efficiently handle operations related to the maximum element, could be an effective way to approach this problem.
It’s also worth noting that the parameter k
essentially controls the number of operations we’re allowed to perform. If k
is smaller than the number of piles, then we won’t be able to perform an operation on every pile. On the other hand, if k
is larger, then some piles may undergo multiple operations.
For the example given, we performed k = 4
operations on our initial 5 piles. If k
were 5, we would perform one more operation, this time on the pile of 9 gifts, leaving 3 behind for updated piles [5,8,3,4,3]
. If k
were only 3, we’d stop one operation earlier, leaving us with piles [25,8,9,4,10]
.
Inference of ProblemSolving Approach from the Problem Statement
Sure, let’s break it down:
Integer Array (gifts): The problem presents us with a collection of data (number of gifts in piles), which is an integer array. This tells us that we’ll be dealing with indexed data where order might be important. For this problem, we don’t care about the initial order of the piles, but we do care about their relative sizes, as we always want to choose the pile with the most gifts. This leads us towards using a data structure that can help us efficiently find and manipulate the maximum element, like a max heap.
Maximum Number of Gifts: The problem requires us to always choose the pile with the maximum number of gifts. This directly implies that we need a way to always identify the maximum element in our collection. A simple list or array would require us to scan the entire collection to find the maximum, which isn’t efficient. A max heap, on the other hand, keeps the maximum element at the top and allows us to efficiently extract and adjust it.
Square Root: The number of gifts we leave behind in a pile is based on the floor of the square root of the number of gifts in that pile. This informs our approach to manipulating the pile’s count. Knowing that we’re working with the square root helps us understand that the pile’s count will decrease significantly with each operation.
k Seconds/Operations: We’re given a fixed number of operations we can perform. This helps define the bounds of our problem and informs us when to stop our procedure. This is essentially a loop that we continue until we exhaust the number of allowed operations.
Remaining Gifts: The goal is to calculate the total number of remaining gifts. This informs us that at the end of our procedure, we’ll need to sum up the gifts left in each pile.
From these key terms, we can see that this problem is about data manipulation (adding and removing elements, finding the maximum) under certain rules (square root rule, fixed number of operations). These clues guide us towards using a max heap as our data structure and implementing a loop to perform the operations.
In terms of visualization, we can look at this problem as dynamically changing states of piles of gifts. Each pile is represented by an element in an array or list. Let’s take an example to illustrate this:
Gifts = [25, 64, 9, 4, 100], k = 4
The state of piles in the beginning:
Pile 1: 25
Pile 2: 64
Pile 3: 9
Pile 4: 4
Pile 5: 100
As we follow the rules, we choose the pile with the maximum number of gifts, subtract the floor of the square root of the number of gifts in that pile, and take the rest of the gifts. Let’s denote the current pile being chosen with (*). After each operation, the state of piles changes:
After 1st operation (choose pile 5 with 100 gifts and leave sqrt(100)=10 gifts):
Pile 1: 25
Pile 2: 64
Pile 3: 9
Pile 4: 4
Pile 5(*): 10
After 2nd operation (choose pile 2 with 64 gifts and leave sqrt(64)=8 gifts):
Pile 1: 25
Pile 2(*): 8
Pile 3: 9
Pile 4: 4
Pile 5: 10
And so on. At each step, you’re looking for the pile with the highest count (max heap property), and then you’re updating that pile according to the rule (sqrt rule).
By doing this, you can visualize the problem as a series of steps, each one updating the state of the piles. This also helps identify the core properties the problem is exploiting: the max heap property of always working with the highest element, and the square root rule of reducing the pile size.
The hints that this problem might be solved using a heap (specifically, a max heap) come from two key points in the problem statement:
You always choose the pile with the maximum number of gifts. This requires an efficient way to find the maximum element in a list, which is a classic use case for a max heap. In a max heap, the maximum element is always at the top and can be retrieved in constant time.
After choosing a pile, you change the number of gifts in it. This implies that the list of piles is not static and will be updated multiple times. After choosing a pile and updating its gift count, you need to restore the maxheap property (i.e., the maximum element is at the top). Heap data structures are designed to allow efficient insertion and deletion of elements while maintaining their heap property, making them suitable for this scenario.
So, the requirements to always choose the maximum element and to frequently update the list of elements both suggest that a max heap could be a good choice for solving this problem efficiently.
Simple Explanation of the Proof
Sure, let’s try to break down the reasoning behind the algorithm:
The problem requires us to keep choosing the pile with the most gifts and take gifts from it, leaving behind the floor of the square root of the current number of gifts. We need to do this process ‘k’ times.
The key to understanding the proof is understanding why a heap data structure (specifically, a max heap) is being used:
Selecting the maximum: We use a max heap because it allows us to always efficiently access and remove the pile with the maximum number of gifts. This is important because the problem statement specifies that we always choose the pile with the maximum number of gifts. In a max heap, the maximum element is always at the top of the heap, allowing us to access and remove it in O(log n) time where n is the number of piles.
Updating the elements: After taking gifts from the chosen pile, the number of gifts in that pile changes, so we need to update our max heap to reflect this change. Max heap is an efficient data structure for this scenario because it allows us to add a new element and maintain the max heap property in O(log n) time.
Repeating the process: The problem statement specifies that we perform this operation ‘k’ times. The max heap ensures that at each step, we’re choosing the correct pile (the one with the most gifts) and updating it efficiently.
Therefore, the proof of this algorithm lies in the properties of the max heap data structure and the requirements of the problem statement. The max heap allows us to efficiently select and remove the pile with the maximum gifts, update the number of gifts in this pile, and repeat this process ‘k’ times.
Stepwise Refinement
Certainly, let’s break it down:
Initialization: We start by initializing a max heap data structure with the values of the array “gifts”. The max heap will automatically organize itself so that the pile with the most gifts is at the root.
Gift Selection and Removal: For each second, up to ‘k’ seconds:
 We take the pile with the maximum number of gifts. This can be achieved by popping the top of the max heap.
 We then calculate how many gifts to leave behind (which is the floor of the square root of the current number of gifts) and take the rest. This involves a simple arithmetic operation and the use of the
math.sqrt()
function in Python.  We then add the number of gifts left in the pile back into the max heap.
Final Calculation: After the ‘k’ seconds are over, we sum up all the remaining gifts in each pile, which can be done by summing all the elements of the max heap.
This approach consists of repeating the same sequence of operations (i.e., selecting the pile with the maximum gifts, taking gifts, and updating the number of gifts in the pile) for ‘k’ times. This repeating pattern makes it wellsuited for a loop structure in our code.
Each ‘k’ iteration is independent of the others, because the choice of pile and the number of gifts taken doesn’t depend on what happened in previous seconds. Instead, it’s determined solely by the current state of the max heap. As a result, we don’t need to maintain any additional state information between iterations, and each iteration can be thought of as solving a small independent subproblem: taking gifts from the currently biggest pile.
Solution Approach and Analysis
Absolutely, let’s break down the approach to solving this problem:
Step 1: Understand the rules of the game Think of this problem as a game, where you are faced with different piles of gifts and each round you can choose to take gifts from the largest pile.
Step 2: Setup the game board To start, we are given an array of integers, where each integer represents the number of gifts in a pile. We will use a max heap to represent these piles. A max heap is a type of binary tree where the parent node is always larger than or equal to its children. In our case, the parent node will always represent the pile with the maximum number of gifts.
Step 3: Play the game The game is played over ‘k’ rounds (seconds). In each round:
 We pick the pile with the maximum number of gifts. This can be done by removing the root node of the max heap.
 We calculate the number of gifts to leave behind in that pile, which is the floor of the square root of the current number of gifts.
 We put the remaining gifts back into the heap.
Step 4: Summarize the game results Once ‘k’ rounds are over, the number of gifts remaining in all the piles is the sum of the elements in the heap.
Now, let’s visualize this with an example:
Consider gifts = [25,64,9,4,100] and k = 4.
Our max heap initially looks like this: [100, 64, 9, 4, 25]. (The actual structure of the heap depends on the specific heap implementation, but the maximum element is always at the top.)
In the first second, we choose the pile with 100 gifts, leave
sqrt(100) = 10
gifts behind, and put 10 back into the heap. The heap now looks like this: [64, 25, 9, 4, 10].In the second second, we choose the pile with 64 gifts, leave
sqrt(64) = 8
gifts behind, and put 8 back into the heap. The heap now looks like this: [25, 10, 9, 4, 8].In the third second, we choose the pile with 25 gifts, leave
sqrt(25) = 5
gifts behind, and put 5 back into the heap. The heap now looks like this: [10, 8, 9, 4, 5].In the fourth second, we choose the pile with 10 gifts, leave
sqrt(10) = 3
(since we take the floor of the square root) gifts behind, and put 3 back into the heap. The heap now looks like this: [9, 8, 5, 4, 3].
So, after 4 seconds, we have a total of 9 + 8 + 5 + 4 + 3 = 29
gifts remaining.
In terms of how the parameters affect the solution:
 The larger the numbers in the ‘gifts’ array, the more gifts you can take before the pile becomes small enough that the square root significantly reduces the number of gifts you can take.
 The larger ‘k’ is, the more times you get to take gifts. However, each time you take gifts, the biggest pile gets smaller. So if ‘k’ is much larger than the size of the ‘gifts’ array, you’ll eventually be taking only one or a few gifts per second, since
sqrt(n)
is 1 for n<4.  The distribution of the numbers in ‘gifts’ also matters. If the numbers are very uneven, you’ll be able to take a lot of gifts from the large piles before you have to start taking from the smaller piles. If the numbers are very even, you’ll quickly reach a point where you’re taking the same small number of gifts from every pile.
Identify Invariant
An invariant in the context of algorithm design is a condition that can be relied upon to be true during execution of a program, or during some portion of it. It’s a logical assertion that is always maintained as true during the life cycle of a program.
In this particular problem, one of the invariants is that at any point of time during the execution of the algorithm, the root of the max heap always contains the pile with the maximum number of gifts.
This property is fundamental to our strategy, as it allows us to efficiently choose the pile from which we take gifts every second. No matter how the contents of the heap change, this property remains true until there are no more elements left in the heap.
Another invariant in this problem is that after each operation, every pile in the heap contains the floor of the square root of the number of gifts it had before the operation. This invariant ensures that we correctly follow the rules of the problem, reducing each pile by the required amount every time we take gifts from it.
These invariants guide the execution of our algorithm and help to ensure that it correctly implements the logic needed to solve the problem.
Identify Loop Invariant
In this problem, the loop invariant, i.e., a condition that holds before and after each iteration of a loop, would be the same as the invariant discussed earlier:
At the start and end of each iteration of the loop (i.e., each second), the root of the max heap contains the pile with the maximum number of gifts.
Before the loop starts, we build a max heap from the piles of gifts, which ensures the heap property holds, i.e., the pile with the maximum gifts is at the root. Then during each iteration, we remove the pile with the maximum gifts, adjust it, and push it back into the heap. Since we’re using a max heap, the heap property is restored after the operation, meaning the pile with the maximum number of gifts is once again at the root.
This loop invariant is critical to our approach, as it allows us to continuously select and operate on the pile with the maximum gifts in an efficient manner.
Yes, in this particular problem, the invariant and the loop invariant happen to be the same.
The term “invariant” often refers to a condition that remains true regardless of the system’s state. For this problem, the invariant is that at any given time, the root of the heap holds the maximum number of gifts.
A “loop invariant”, on the other hand, is a condition that is initially true and remains true after each iteration of a loop. In this case, our loop is the process of taking gifts every second, and the loop invariant is the same as our general invariant: that the root of the heap contains the maximum number of gifts at the start and end of each loop iteration.
So in the context of this problem, our invariant and loop invariant refer to the same condition, but it’s important to note that this isn’t always the case in other problems. It depends on the specifics of the problem at hand.
Thought Process
Sure, here’s a stepbystep approach:
Step 1: Understand the Problem The problem describes an operation on a list of integers that represent piles of gifts. Every second, you choose the pile with the most gifts, and you replace the number of gifts in that pile with the floor of its square root. You continue doing this operation for k seconds.
Step 2: Identify Key Concepts
 The problem involves a list of integers and a number of operations to perform.
 You always choose the pile with the maximum number of gifts.
 You replace the number of gifts in the pile with the floor of its square root.
 These operations need to be performed quickly because you’ll do them for a potentially large number of seconds.
Step 3: Formulate Approach With the identified key concepts, we can see that this problem might be solved efficiently using a priority queue (max heap). A max heap can provide the maximum element in constant time, and remove/insert operations in O(log n) time. This aligns well with the need to always choose the maximum element and then update it.
Here is the approach based on the above:
 Initialize a max heap with the initial list of gifts. This takes O(n log n) time, where n is the number of piles.
 Repeat the following process k times:
 Extract the maximum element from the heap. This is the pile with the most gifts.
 Compute the floor of the square root of the maximum element.
 Insert the result back into the heap.
Now let’s write this in Python:


This code should work for the problem as specified. It uses a heap data structure from Python’s heapq module to efficiently find and remove the maximum element in the gifts list. It then computes the remaining gifts in the chosen pile and adds it back to the heap, repeating this process k times. Finally, it returns the sum of remaining gifts in all piles.
In terms of efficiency, creating the heap takes O(n log n) time, and we perform k operations that each take O(log n) time, giving a total time complexity of O(n log n + k log n).
Establishing Preconditions and Postconditions
Parameters:
 The method takes two parameters:
gifts
andk
. gifts
is a list of integers where each integer represents the number of gifts in a pile.k
is an integer that represents the number of seconds.gifts
represents the initial state of gifts across different piles andk
represents the time duration in seconds for which the process of picking gifts continues.
 The method takes two parameters:
Preconditions:
 Before this method is called, there are no specific conditions that must be true about the state of the program.
 The constraints on the parameters are: 1 <= gifts.length <= 10^3, 1 <= gifts[i] <= 10^9, and 1 <= k <= 10^3.
 There is no specific state the program must be in.
Method Functionality:
 This method is expected to simulate the process of picking gifts from the piles for
k
seconds. At each second, it picks the pile with the maximum gifts, takes the majority of them, and leaves behind the floor of the square root of the number of gifts in that pile.  The method interacts with the input parameters to simulate this process and calculate the remaining number of gifts after
k
seconds.
 This method is expected to simulate the process of picking gifts from the piles for
Postconditions:
 After the method has been called and returned, the state of the program doesn’t change as we are not modifying any global state.
 The return value represents the total number of gifts remaining after
k
seconds.  The method does not have any side effects.
Error Handling:
 The method assumes that the preconditions are met, i.e., the parameters follow the mentioned constraints. If the preconditions are not met, the behaviour of the method is undefined.
 The method does not explicitly handle errors or throw exceptions. If invalid inputs are provided, it might result in unexpected outputs or runtime errors.
Problem Decomposition
Problem Understanding:
 The problem involves choosing the largest pile of gifts every second, taking the majority of them, and reducing the pile size to the square root of its original size. This process continues for
k
seconds. The key requirement is to calculate the total number of gifts remaining afterk
seconds.
 The problem involves choosing the largest pile of gifts every second, taking the majority of them, and reducing the pile size to the square root of its original size. This process continues for
Initial Breakdown:
 Major parts of the problem include:
 Identifying the largest pile every second
 Updating the pile size after selecting the gifts
 Repeating the above two steps for
k
seconds  Calculating the total number of gifts remaining
 Major parts of the problem include:
Subproblem Refinement:
 Identifying the largest pile can be further broken down into iterating over the list of piles and finding the maximum.
 Updating the pile size involves calculating the square root of the current size and flooring the result.
 Repeating the steps involves a loop structure where the above operations are executed.
 Calculating the total gifts remaining involves iterating over the piles and adding up their sizes.
Task Identification:
 Tasks like identifying the maximum and iterating over the list to calculate the sum are repeated and can be generalized.
Task Abstraction:
 Each task is abstracted to a sufficient degree that it is clear and can be potentially reusable in similar problems.
Method Naming:
 Potential method names could be
identify_max_pile
,update_pile_size
,execute_gift_picking
for the loop operation, andcalculate_remaining_gifts
for the final calculation.
 Potential method names could be
Subproblem Interactions:
 The
identify_max_pile
andupdate_pile_size
tasks are dependent and need to be performed in order within theexecute_gift_picking
operation. This operation in turn is performedk
times. Finally, thecalculate_remaining_gifts
operation is performed to yield the result.
 The
From Brute Force to Optimal Solution
The brute force approach for this problem would involve a straightforward translation of the problem statement into code.
 Brute Force Solution:


This approach simply iterates over all piles k
times to find the largest pile, updates its size, and finally sums up the remaining gifts in all piles.
However, the inefficiency lies in the fact that we are traversing all piles to find the maximum every second. If we have n
piles and want to continue for k
seconds, this results in a time complexity of O(n*k), which could be quite high for large inputs.
 Optimization:
One possible optimization is to avoid traversing all piles every second to find the max. This can be achieved by initially sorting the piles and then efficiently finding the max, such as by using a binary heap.
A binary heap allows us to extract the maximum element (in the case of a maxheap) or the minimum element (in the case of a minheap) in constant time, i.e., O(1). Adding an element or heapifying takes logarithmic time, i.e., O(log n).


In the optimized solution, we first create a max heap from the piles, with each pile negated because Python’s heapq
library only creates minheaps. We then pop the maximum pile (the heap root) and push the updated pile size back into the heap. This process is repeated k
times, and then we sum up the remaining gifts.
This approach has a time complexity of O(n + klog n): O(n) for creating the heap and O(klog n) for k
heap operations. This is an improvement over the brute force solution for large inputs. The space complexity remains O(n) for storing the heap.
Code Explanation and Design Decisions
Initial Parameters:
piles
is a list of integers representing the size of each pile of gifts. This is the primary input to our function and sets the problem’s landscape, dictating which piles we can reduce to gather more gifts.k
is an integer representing the number of seconds we have to perform our operation of reducing the biggest pile. This is our time constraint, and the total number of operations we can perform.
Primary Loop:
 The primary loop in our solution iterates
k
times. Each iteration represents a single second passing where we perform our operation: find the biggest pile of gifts and reduce it to its square root.
 The primary loop in our solution iterates
Conditions/Branches:
 There are no explicit condition checks (like
if
statements) in our main loop. The action of finding and reducing the biggest pile is performed unconditionally for every second, up tok
seconds.
 There are no explicit condition checks (like
Updates/Modifications:
 Within each loop, the largest pile of gifts is reduced to its square root. This is the core operation that we perform in order to increase the total number of gifts we can collect. The updated pile is then reinserted back into the heap to maintain the maxheap property.
Invariant:
 The invariant in this code is the property of the maxheap. Throughout the iterations, the heap is always arranged such that the largest pile is at the root. This invariant is critical as it allows us to efficiently find and reduce the largest pile in each operation, regardless of the changes we make to the piles.
Final Output:
 The final output of the function is the sum of all the elements in the maxheap, which represents the total number of gifts collected from all piles. This output satisfies the problem’s requirements, as our aim is to maximize the total number of gifts we can gather within the
k
seconds.
 The final output of the function is the sum of all the elements in the maxheap, which represents the total number of gifts collected from all piles. This output satisfies the problem’s requirements, as our aim is to maximize the total number of gifts we can gather within the
Coding Constructs
Highlevel problemsolving strategies:
The code is primarily using the strategy of greedy algorithms, along with a data structure known as a max heap. The greedy part comes from always choosing the largest pile to operate on, with the belief that this choice will lead to the highest number of gifts in the end. The max heap is used to keep the piles of gifts in an order that allows us to quickly and efficiently always find the largest pile.
Explaining to a nonprogrammer:
Imagine you have different piles of gifts and only a limited time to open them. However, every time you choose a pile, you must reduce its size to its square root. Now, you want to get as many gifts as possible in the time you have. So, this “program” is like a smart strategy that always tells you which pile to choose at each moment to maximize your total gifts.
Logical elements or constructs:
Independent of any programming language, the code uses the construct of a loop, running a fixed number of times (
k
seconds). It also uses the principle of a max heap data structure where the maximum element can be efficiently extracted.Algorithmic approach in plain English:
You start by arranging all your piles in a particular way (creating a max heap) that lets you quickly find the biggest pile. Then, for each second up to your limit, you do the following: find the biggest pile, open some gifts from it (reduce it to its square root), and put the remainder back. After all the time has passed, you simply add up all the gifts you’ve opened.
Key steps or operations:
 Creating the max heap from the input piles. This step lets us easily identify the largest pile.
 Running a loop for
k
iterations. Each iteration represents a second passing.  In each iteration, removing the maximum element (pile) from the heap, reducing it to its square root, and adding the reduced pile back to the heap.
 Finally, summing up all the elements in the heap to calculate the total gifts.
Algorithmic patterns or strategies:
The main algorithmic pattern used here is the Greedy algorithm, along with heap manipulation operations. The greedy strategy is used as we’re always choosing the largest pile to operate on (hence, being ‘greedy’). The heap operations (extractmax and insert) are used to efficiently identify and operate on the largest pile.
Language Agnostic Coding Drills
Dissecting the Code into Distinct Concepts:
 Understanding Data Structures: Recognizing the need for a data structure to store piles and their sizes efficiently. In this case, the most appropriate structure is a heap.
 Loop Constructs: Using a loop to repeat a certain action (extracting elements from the heap) a set number of times.
 Mathematical Operations: Calculating the square root of a number.
 Data Structure Manipulation: Inserting and removing elements from the heap.
 Greedy Algorithms: Making the locally optimal choice at each step with the hope that these local choices lead to a global optimum.
Coding Concepts or Drills in Order of Increasing Difficulty:
 Mathematical Operations (Easy): Calculating the square root is a fundamental operation in most programming languages and should be easy to understand.
 Loop Constructs (Easy): Understanding loops is an early learning stage in most programming languages, as they’re often used to perform repeated tasks.
 Understanding Data Structures (Medium): While understanding the concept of data structures might not be too difficult, choosing the right one for a particular problem can be challenging.
 Data Structure Manipulation (Medium): Once the appropriate data structure is chosen, understanding how to manipulate it (insertion, deletion, accessing elements) can also pose a challenge.
 Greedy Algorithms (Hard): Understanding and applying greedy algorithms can be challenging, as it requires understanding the problem’s context and constraints and determining if making locally optimal choices will lead to a global optimum.
ProblemSolving Approach From Problem Statement to Final Solution:
 Start with the problem statement and identify the main goal: to maximize the number of gifts within a given time frame. This hints towards a greedy approach since we’re trying to optimize a certain parameter.
 Recognize that we are dealing with ‘piles’ of gifts, suggesting the need for a data structure to hold these collections. The fact that we always want to choose the largest pile to maximize gifts suggests that a heap data structure would be efficient, as it allows for quick access to the maximum element.
 Set up a loop to simulate the passage of time (
k
seconds). This is where the concept of loop constructs comes in.  Within this loop, use the greedy approach to always choose the pile with the most gifts. This is where the heap comes into play, as it allows us to efficiently find and remove the maximum element.
 After selecting the largest pile, we reduce its size by taking the square root, which introduces the concept of mathematical operations.
 The reduced pile is then added back into the heap (data structure manipulation).
 Once all the time has passed (the loop has run its course), we are left with the remaining piles in the heap. Summing these up gives us the total gifts obtained.
By breaking down the problem into these coding drills, we can gradually build up to the final solution, ensuring that each concept is understood before moving on to the next. This approach allows for a comprehensive understanding of both the problem and the solution.
Targeted Drills in Python
Individual Python Coding Drills for Each Identified Concept:
Mathematical Operations (Calculating the Square Root):
1 2 3 4
import math def calculate_square_root(num): return math.isqrt(num)
Loop Constructs:
1 2 3
def loop_n_times(n): for i in range(n): print(f"This is iteration number {i+1}")
Understanding Data Structures (Using a Heap):
1 2 3 4 5 6 7
import heapq def heap_operations(): heap = [] # Initialize an empty list to be used as a heap heapq.heappush(heap, 5) # Insert 5 into the heap heapq.heappush(heap, 3) # Insert 3 into the heap print(heapq.heappop(heap)) # Remove the smallest element and print it
Data Structure Manipulation (Manipulating a Heap):
1 2 3 4 5 6 7 8
import heapq def manipulate_heap(): heap = [5, 3, 8, 1] # Initialize a heap with some elements heapq.heapify(heap) # Convert the list into a valid heap heapq.heappush(heap, 10) # Insert 10 into the heap print(heapq.heappop(heap)) # Remove the smallest element and print it print(heap[0]) # Print the smallest element without removing it
Greedy Algorithms (Using a Heap to Implement a Greedy Approach):
1 2 3 4 5 6 7 8 9 10 11 12 13 14
import heapq def greedy_algorithm(): items = [5, 3, 8, 1] # Initialize a list of items (negative for max heap) heapq.heapify(items) # Convert the list into a valid heap total = 0 # Initialize a counter to keep track of the total while items: # Continue until there are no more items item = heapq.heappop(items) # Remove and return the largest item total += item # Add the item to the total if item > 1: # If there's more than one unit left heapq.heappush(items, item // 2) # Half the item and put it back print(total) # Print the total
ProblemSpecific Python Coding Drills:
The given problem is fairly general and does not involve any very specific or unusual concepts that would require additional drills. The concepts covered above, when understood and applied properly, should be sufficient to solve the problem.
Integrating the Drills to Solve the Problem:
 First, use the ‘Understanding Data Structures’ drill to create a heap from the given list of piles.
 Then, use the ‘Loop Constructs’ drill to set up a loop that will run for
k
iterations.  Within this loop, use the ‘Greedy Algorithms’ and ‘Data Structure Manipulation’ drills to select the largest pile, remove it from the heap, and add half its size back to the heap.
 Use the ‘Mathematical Operations’ drill to calculate half the size of the pile.
 Finally, sum up the sizes of the remaining piles in the heap for the final result.
Identifying Problem Isomorphism
The problem “Take Gifts from the Richest Pile” can be mapped to the problem “Last Stone Weight”.
Both problems have a similar structure where we continually pick the largest items from a collection, perform an operation on them, and then put them back into the collection until a certain condition is met.
In “Take Gifts from the Richest Pile”, we repeatedly pick the largest gift, take the square root of its value, and return it to the collection for ‘k’ times. In “Last Stone Weight”, we pick the two heaviest stones, smash them together (subtract the weights), and return the result back into the collection until there is only one stone left or no stones at all.
Both problems also involve the use of a data structure like a maxheap or priority queue to efficiently find and extract the largest items in the collection.
The difference lies in the specific operations being performed and the end conditions, but the overall problemsolving strategy and structure are quite similar. If you understand how to solve one, you can use similar techniques and thought processes to solve the other.
In terms of difficulty, “Last Stone Weight” is typically considered simpler as it has less operations and conditions to manage compared to “Take Gifts from the Richest Pile”.
Q&A
you always look at the array and reconsider ALL values, INCLUDING the values that you’ve already handled.
gifts = [25,64,9,4,100]
k = 1, max is 100, so we replace it with sqrt(100) gifts = [25,64,9,4,10]
k = 2, max is 64, so we replace it with sqrt(64) gifts = [25,8,9,4,10]
k = 3, max is 25, so we replace it with sqrt(25) gifts = [5,8,9,4,10]
THIS IS WHERE IT’S CONFUSING: k = 4, max is ..not 9, it’s the value we modified at step k = 1, aka 10, so we replace this max value with floor of sqrt(10) = 3 gifts = [5,8,9,4,3]
Now add all vals, and you get 5 + 8 + 9 + 4 + 3 = 29
The confusion seems to stem from the fact that the array is being modified in each iteration and the maximum value is reconsidered in every step. The key here is that we are always looking for the maximum gift in the pile, even if that pile has been modified in a previous step.
In your example, when k
equals 4, the maximum value is indeed 10, not 9. Even though the value of 10 came from taking the square root of a previously larger value (100), it’s still the largest value in the array at the time of the fourth step.
Remember, the strategy is to always take gifts from the richest pile. So, we will always be looking for the maximum value in the array, regardless of whether that pile has been reduced previously.
After all the steps are completed, we simply add all the values in the gift piles to get the final sum, which in this case is 29. The confusion might arise from the continuous modification of the array, but it is key to solving this problem as the problem statement guides us to always choose the richest pile.
Two approaches that readily come to mind:
Brute force: Sorting gifts[] before every operation Priority Queue (Reverse Order)
You’re correct. Here’s a bit more detail about both approaches:
Brute Force Approach: The brute force approach involves sorting the array before every operation.
Steps:
 Sort the gifts array in descending order.
 Perform the operation on the first element (since it’s the maximum).
 Repeat these steps until you’ve performed ‘k’ operations.
The issue with this approach is its inefficiency. If there are ’n’ elements in the array and you’re performing ‘k’ operations, the time complexity would be O(n log n * k) because you’re sorting the array before every operation.
Priority Queue Approach: A more efficient approach would be to use a priority queue, specifically a maxheap.
Steps:
 Insert all elements into a maxheap.
 Extract the maximum element, perform the operation, and insert it back into the heap. Repeat this ‘k’ times.
 Finally, sum up all elements in the heap.
The time complexity for building the heap is O(n), for performing ‘k’ operations it’s O(k log n), and for summing up it’s O(n), so overall it’s O(n + k log n). This is a significant improvement over the brute force approach, particularly when ‘k’ is large.
Both these approaches are applicable, but the choice between them depends on the constraints of the problem – if ‘k’ is large, using a priority queue will be more efficient.
Could someone please tell me what is wrong with my solution? It’s working but only for a few cases…
while k != 0:
for i in range(len(gifts)):
x = math.floor(math.sqrt(max(gifts)))
if gifts[i] == max(gifts):
gifts[i] = x
k = 1
return gifts
Your approach seems to have a nested loop which results in the operation executing len(gifts) * k
times. However, you need the operation to only run ‘k’ times. The current structure of the code causes it to modify the pile ‘k’ times for each pile, which is not what the problem statement specifies.
Additionally, you’re finding the maximum value of the array in every iteration, which is a heavy operation and could be optimized.
Instead, what you need is a single loop that runs ‘k’ times. In each iteration, you find the largest pile, take its square root, and replace the original value. One way to efficiently do this is to use a priority queue (heap) data structure which keeps the elements in order and allows efficient extraction of the maximum element.
I do not understand whats wrong with my solution. It fails on test 52.


The issue here is likely with how you’re building your initial max heap. The max heapify function is being called starting from the last leaf node’s parent and you do not iterate backwards to include all parents of the leaf nodes. Because of this, your initial heap might not be a max heap.
When you start from the last leaf node’s parent, you should then go backwards through the array, ensuring all subtrees are max heaps. You can do this by adding a loop that goes from last leaf node’s parent to the root of the tree (index 0) in a reverse manner.
Here’s how you can modify the max heap construction part of your code:


This way, you ensure that your heap property is maintained from the start. Then, as you continue to pick gifts, the max heapify function should correctly rearrange the elements to maintain the max heap property.
This change is necessary because in the heap data structure, the parent node must always have a value that is greater than or equal to its children. So, we need to start heapifying from the bottom of the tree and work our way up to the root to ensure this property is maintained.
Similar to https://leetcode.com/problems/maximalscoreafterapplyingkoperations/
The problem “Maximal Score After Applying K Operations” is indeed similar to “Take Gifts From The Richest Pile”. Both problems are centered around applying operations to a collection of elements to maximize a certain objective. Here’s why they map well to each other:
Operations on a Heap: Both problems involve a heaplike structure, or at least, can benefit from the use of a heaplike structure. This allows you to always select the maximum element in the collection, and perform the required operation.
Limited Operations: In both problems, you are allowed a limited number of operations (K operations) to reach your goal.
Transformation Rules: Both problems have a transformation rule that’s applied to the element you pick (taking the square root in the case of “Take Gifts From The Richest Pile” and dividing by 2 in “Maximal Score After Applying K Operations”).
Goal of Maximization: Lastly, both problems share the common goal of maximizing the sum after all operations are performed.
In terms of complexity, “Maximal Score After Applying K Operations” can be considered a bit simpler, since its transformation operation (division by 2) is simpler than taking the square root, and it also includes the additional constraint that each number can be halved at most once, which simplifies the solution approach.
whats is wrong with this approach??


This approach has several issues:
Finding Maximum Incorrectly: The loop is intended to find the maximum element in
gifts[]
and then update the maximum element with its square root. However, it’s incorrectly updatinggifts[i]
with the square root ofmaxi
every time it finds a new maximum. As a result, it’s possible that not just the maximum element but several elements get updated in each iteration of the outer loop.Wrong Transformation Operation: According to our original problem, each chosen gift’s value should be reduced to half of its original value, but this code is replacing the maximum value with its square root, which doesn’t match the problem’s requirements.
No Breaking Condition: There is no condition to break out of the loop if all elements in
gifts[]
become 0. The loop continues fork
times, which could lead to unnecessary computations.Time Complexity: This code has a time complexity of O(k*n) because for each of
k
iterations, it’s performing an O(n) operation to find the maximum value in the array. This would not be efficient for large inputs.
Here’s the correct approach:


In this code, max_element()
is a builtin function in C++ that returns an iterator pointing to the maximum element in the range [first, last) that we’ve passed. We then subtract gifts.begin()
from this iterator to get the index of the maximum element in our array. We can then directly halve the value at this index. The loop runs for k
iterations, and in each iteration, we’re finding and halving the maximum value in our array.
Similar Problems
Here are 10 problems that use similar underlying concepts:
Kth Largest Element in an Array (215): This problem requires finding the kth largest element in an unsorted array, which can be efficiently solved using a heap data structure. It aligns with our original problem due to the utilization of heaps and kth largest concept.
Top K Frequent Elements (347): The goal is to find the k most frequent elements in an array. This task is similar to our original problem because it uses a heap to handle the “kth most” paradigm.
Find K Pairs with Smallest Sums (373): In this problem, you must find k pairs with the smallest sums from two arrays. The technique of using a heap to keep track of the smallest sums is analogous to how we tracked the largest piles in our original problem.
Kth Smallest Element in a Sorted Matrix (378): This problem requires us to find the kth smallest element in a sorted matrix. It’s similar to our original problem as it requires an understanding of heaps and the “kth smallest” concept.
Sliding Window Maximum (239): This problem involves using a heap to maintain the maximum of every “window” of numbers as you slide through an array, analogous to keeping track of the maximum pile of stones.
Last Stone Weight (1046): This problem involves a similar scenario where stones (or tasks) are consumed or altered in each step, and the largest one is selected each time, matching the usage of heap data structures.
Trapping Rain Water II (407): This is a more complex problem that still uses heap structures. The heap is used to keep track of the boundary with the lowest height, similar to how we kept track of the largest pile.
Minimum Cost to Connect Sticks (1167): In this problem, you have to combine sticks in a way that minimizes the total cost, which mirrors the way we had to reduce piles in a way that maximized the sum of the stones.
Split Array into Consecutive Subsequences (659): This problem requires using a greedy algorithm to split an array into as many subsequences of consecutive numbers as possible. The greedy algorithmic approach is similar to our original problem.
Reorganize String (767): The problem involves rearranging a string in a way that no two same characters come together. This problem’s approach is akin to the way we had to wisely choose piles to maximize our sum, with the condition to reduce the largest pile by half at each step.