Maximum Number of Events That Can Be Attended II
Identifying Problem Isomorphism
“Maximum Number of Events That Can Be Attended II” shares some similarities with “Knapsack Problem”.
Reasoning:
In “Maximum Number of Events That Can Be Attended II”, you are given a series of events, each with a starting and ending day and a value. You can choose to attend certain events and cannot attend two events that overlap in time. The goal is to maximize the total value of the events attended, but you can attend at most k events.
In “Knapsack Problem”, you are given a set of items, each with a weight and a value, and a maximum weight capacity for a knapsack. You can choose to include certain items and cannot include two items if their combined weight exceeds the capacity. The goal is to maximize the total value of the items included.
Both involve making decisions to maximize a total value subject to constraints (event time overlap in the first problem, knapsack weight capacity in the second) and an overall limit (maximum number of events in the first problem, maximum weight in the second).
Both require a dynamic programming solution to reach an optimal outcome, but the “Knapsack Problem” is simpler due to its familiarity. It deals with a single, tangible constraint (weight), whereas “Maximum Number of Events That Can Be Attended II” deals with multiple events, each having different start and end times, which can be seen as more complex.
10 Prerequisite LeetCode Problems
“1751. Maximum Number of Events That Can Be Attended II” combines the concepts of sorting, greedy algorithms, and dynamic programming. Here are some simpler problems to prepare for this:
LeetCode 646. Maximum Length of Pair Chain
 This problem introduces the concept of sorting pairs and making decisions to maximize the chain length.
LeetCode 376. Wiggle Subsequence
 This problem will help you understand the dynamic programming concept of maintaining a maximum count.
LeetCode 435. Nonoverlapping Intervals
 This problem is about choosing intervals in a way that maximizes the count of nonoverlapping ones.
LeetCode 628. Maximum Product of Three Numbers
 This problem will help you understand the greedy concept of making local decisions to achieve a global maximum.
LeetCode 300. Longest Increasing Subsequence
 This problem introduces dynamic programming for maintaining the longest sequence.
LeetCode 354. Russian Doll Envelopes
 This problem requires you to sort the envelopes and then find the longest increasing subsequence, a similar concept to the “Maximum Number of Events That Can Be Attended II” problem.
LeetCode 198. House Robber
 This problem is a classic dynamic programming problem where you make a decision at each house to rob or not.
LeetCode 213. House Robber II
 This problem extends the House Robber problem with a circular condition.
LeetCode 1043. Partition Array for Maximum Sum
 This problem involves partitioning an array for a maximum sum, a similar concept to attending events in the “Maximum Number of Events That Can Be Attended II” problem.
LeetCode 1105. Filling Bookcase Shelves
 This problem requires you to make decisions to minimize the height, a similar concept to maximizing the number of events in the “Maximum Number of Events That Can Be Attended II” problem.
These cover how to use sorting, greedy algorithms, and dynamic programming to solve problems, which are key to solving the “1751. Maximum Number of Events That Can Be Attended II” problem.
Problem Classification
This problem falls within the Scheduling domain. It involves determining a schedule for attending events, given restrictions on the number of events that can be attended and the fact that only one event can be attended at a time. It’s a type of optimization problem where the goal is to maximize the total value received from attending events.
What components:
Events: These are described by a start day, an end day, and a value. The start and end days denote when the event begins and ends, respectively. The value is a quantity you receive if you attend the event.
k: This is a constraint that specifies the maximum number of events you can attend.
Event Overlapping: The problem states that you can only attend one event at a time, meaning that if you choose to attend an event, you cannot attend any other event that starts or ends on the same day.
Event Selection: The goal is to select a set of events to attend that maximizes the total value received, subject to the constraints on the number of events that can be attended and the fact that event times cannot overlap.
This problem can be classified as a Greedy Algorithm or Dynamic Programming problem. It’s about making optimal decisions given a set of constraints, where the decision to attend an event impacts future decisions (since it precludes attending overlapping events). It involves considering all possible choices (which events to attend) and selecting the one that results in the maximum total value. Therefore, it can be solved either by making the locally optimal choice at each step (Greedy Algorithm approach) or by solving it in a bottomup manner, breaking it down into simpler subproblems and building up to the solution (Dynamic Programming approach).
Visual Model of the Problem
For this problem, a helpful way to visualize the problem statement could be to consider a timeline representing the days, with each event marked as a segment along this timeline. The length of each segment represents the duration of the event, from the start day to the end day, and you could annotate each segment with the event’s value. This visualization would allow you to easily see when each event starts and ends, as well as overlapping events.
Here is a textual representation for the first example:
Days: 1 2 3 4
Event1: 
Event2: 
Event3: 
Values: 4 3 1
In the above representation, the hyphens represent the duration of each event on the timeline. For example, Event 1 starts on day 1 and ends on day 2 and has a value of 4. Event 2 starts on day 3 and ends on day 4 and has a value of 3. Event 3 starts on day 2 and ends on day 3 and has a value of 1.
This type of visualization can help to see which events overlap and therefore cannot be attended together, and it also helps to visualize which combinations of events could be attended to achieve the maximum total value, considering the constraint of maximum k events that can be attended.
In more complex examples, a graphical visualization could be more effective, allowing for a clear view of overlaps and the capacity to compare event values more effectively.
Clarification Questions
When trying to fully understand a problem statement, there can be several important points that need to be clarified. For this specific problem, here are some clarification questions that one might ask:
 What should be returned if there are no events? Should we return 0 or null?
 Can the start and end day of an event be the same, i.e., can the event last for only one day?
 Is the array of events sorted in any particular order, such as by start day or end day or value? Or can the events be in any arbitrary order?
 Are there any constraints on the values of start day, end day, or the event’s value?
 Can two events start on the same day?
 Can two events end on the same day?
 Are the events guaranteed to be feasible, i.e., the end day is always equal or after the start day?
 If there are more than ‘k’ events with the same value and duration, is there a specific selection preference?
 If the maximum sum of values can be achieved in multiple ways, do we need to return all possible sets of events or just one?
Remember, the goal of asking clarification questions is to eliminate assumptions that might lead to incorrect solutions.
Problem Restatement
We are given a list of events, each having a start day, an end day, and a value. Attending an event means being present for its full duration, from start to end day. Furthermore, we can only attend one event at a time, meaning overlapping events cannot be attended simultaneously. Given the maximum number of events we can attend, our task is to maximize the total value obtained from attending these events.
In other words, we need to choose which events to attend, taking into consideration their timeframes and the values they provide, with the aim to maximize the sum of the chosen event values. However, the total number of events attended should not exceed the maximum limit provided to us.
Remember, the end day of an event is inclusive; this means that we cannot attend two events where one starts and the other ends on the same day. Additionally, it’s not required to attend all ‘k’ events if doing so doesn’t result in the highest value. The objective is to maximize the value, even if it means attending fewer than ‘k’ events.
Abstract Representation of the Problem
We are given a set of intervals, each having a start point, an endpoint, and a weight. The weight represents the benefit or value obtained from participating in the interval. Each interval requires full participation, from start to end. Furthermore, we can only participate in one interval at a time, implying that overlapping intervals are mutually exclusive.
We are also provided with an upper limit to the total number of intervals we can participate in. The task is to select a subset of intervals to participate in, with a maximum size of the given limit, such that the sum of their weights is maximized.
It’s essential to note that the endpoints of intervals are inclusive, which means that two intervals with adjacent end and start points are considered overlapping. Additionally, it’s not necessary to utilize the entire maximum limit if it doesn’t lead to maximum weight. The goal is to maximize the total weight, even if it means participating in fewer intervals than the upper limit.
This problem lies at the intersection of interval scheduling and knapsack problem, where we are to make an optimal selection of weighted intervals under given constraints.
Terminology
Interval: An interval in this context is a time period that begins at a certain point (start day) and ends at another point (end day). Intervals can overlap, meaning they share some days.
Weight: The “weight” of an interval in this problem is the value you get for attending an event. It’s the benefit or payoff associated with each interval.
Overlapping Intervals: Two intervals are said to be overlapping if they share some common time. In this problem, if one event’s end day is the same as the next event’s start day, they’re considered overlapping, and you can’t attend both.
Nonoverlapping (or mutually exclusive) intervals: These are intervals that don’t share any time with each other. You can attend both of these events.
Interval Scheduling: This is a class of problems in computer science where you want to select a maximumsize subset of mutually nonoverlapping intervals.
Knapsack Problem: This is a problem in combinatorial optimization. You’re trying to maximize the total value of items you can “pack” given a certain capacity (in this case, the number of events you can attend). You can’t break items down; you either take the whole item or don’t take it at all. In this problem, each “item” is an event, and the “value” is the event’s value.
Optimal Selection: This term refers to the process of making the best or most effective choice out of some set of options given certain constraints. In this problem, the optimal selection refers to choosing the events to attend to maximize the total value.
In this problem, we are dealing with a form of the interval scheduling problem but with an additional weight associated with each interval, which is why it is also related to the knapsack problem. The weight or value we gain from each interval adds an extra dimension to the problem, making it more challenging to solve.
Problem Simplification and Explanation
Let’s consider this problem in the context of a music festival, where each band or artist’s performance is an “event”.
Suppose you’re at a music festival with multiple stages. Each band has a set time they’ll be playing, from start to end, and you’re given a schedule that tells you when each band will be playing. The catch is that some bands might be playing at the same time on different stages. In addition, you have a personal preference for each band, meaning you’ll enjoy seeing some bands more than others  this is the “value” associated with each band’s performance.
Now, you also have a limit to the number of bands you can see because you get tired after watching a certain number of performances. The question is, how do you decide which bands to watch to maximize your enjoyment of the festival, given that you can’t see more than a certain number of bands, and you can’t see two bands that are playing at the same time?
The bands are the events, the times each band is playing are the start and end times, and the limit to the number of bands you can see is the number k. The ‘value’ you get from watching each band is the event value in the problem.
The key concepts involved here are:
 Selection: Out of all the bands playing, which ones should you choose to maximize your total enjoyment?
 Scheduling: The timing of each band’s performance matters. You can’t be in two places at once, so you need to make sure the bands you select don’t have overlapping set times.
 Constraints: You have a maximum number of bands you can see (k), and you need to operate within this limit.
Breaking it down, the problem is essentially about making the best choice from a set of options, given certain constraints. You need to make a selection that accounts for both the value of each option and how the options fit together in terms of scheduling.
Constraints
Here are some specific characteristics or conditions that can be exploited for an efficient solution:
Predefined Limits: The constraints on the number of events, their durations, and their values are all reasonably limited (up to 10^6, 10^9, and 10^6, respectively). This suggests that certain algorithms like sorting the events, or using data structures like heaps or binary search trees, can be used without worrying too much about efficiency.
Overlapping Events: The fact that we can only attend one event at a time implies that we must deal with overlapping events. This can be handled by sorting the events by their end times and then applying a greedy algorithm to select the most valuable event that doesn’t conflict with the current schedule.
Limited Attendance: We are limited to attend ‘k’ events. This means we cannot just attend all events, but instead must make a strategic choice. Dynamic programming or other optimization techniques can be useful here to make the best choice at each step.
Event Value: Each event carries a value, which can be considered as the benefit we get from attending the event. This introduces a decision factor in our problem. A higher value event is more preferable, but it’s also crucial to consider the duration of the event since longer events may prevent us from attending other valuable events. Hence, a ratio of value per duration could be a valuable metric in our decision process.
Nonoverlapping Events: The problem states that we cannot attend two events if one starts and the other ends on the same day. This means we will need to include an additional check in our algorithm to ensure that we don’t attend two events on the same day.
These characteristics can be used to guide the choice of data structures and algorithms that we use in our solution. For example, we might use a priority queue to select the most valuable events, or a sorting algorithm to ensure that we consider events in the correct order. We might also use dynamic programming or a greedy algorithm to handle the constraints on the number of events we can attend and the nonoverlapping condition.
Analyzing the constraints of this problem can provide several key insights that help in designing an efficient solution:
Number of Events (1 <= k <= events.length, 1 <= k * events.length <= 10^6): These constraints imply that the total number of events is fairly manageable, which allows us to consider algorithms that are not necessarily linear in time complexity.
Event Duration (1 <= startDayi <= endDayi <= 10^9): Event durations could be quite large, but importantly, they are finite and well defined. This suggests that we cannot simply consider all possible subsets of events, as the number of such subsets would be astronomically large. Instead, we need to devise a strategy that intelligently selects which events to attend.
Event Values (1 <= valuei <= 10^6): Event values are positive and can be large, but they are also reasonably bounded. This means that we can use these values directly in our computations, without needing to normalize or scale them.
Overlapping Events: The constraints do not prevent events from overlapping, and in fact, we are explicitly told that we cannot attend two events if one starts and the other ends on the same day. This suggests that handling overlapping events is likely to be a key part of the solution.
Limited Event Attendance: We can attend at most ‘k’ events, suggesting that we need to prioritize events based on some criteria, such as their values or duration. This prioritization process is likely to involve sorting or a priority queue.
Objective: The goal is to maximize the total value gained from attending events. This indicates that our problem is an optimization problem, which could suggest strategies like dynamic programming or greedy algorithms.
These insights can help inform the design of an efficient algorithm for the problem, guiding the selection of appropriate data structures and techniques.
Problem Analysis and Key Insights
Upon analyzing the problem statement, a few key insights can be derived:
Event Selection: The problem is essentially about making a choice from a set of options (the events) to maximize the total value within a certain constraint (attending up to ‘k’ events).
NonOverlapping: You cannot attend two events that overlap on any day, so we need to carefully select events such that they do not clash. This requires the use of scheduling algorithms to ensure that there are no overlaps.
Maximize Value: The ultimate goal is to maximize the total value. This suggests that the solution might require a greedy or dynamic programming approach.
Number of Events: The problem statement mentions that we can attend at most ‘k’ events but does not mandate that we must attend ‘k’ events. So, the solution must handle situations where attending less than ‘k’ events could give a higher total value.
Event Duration: The duration of an event does not affect its value; an event could last one day and have a high value, while another could span several days and have a lesser value. Hence, the valuetoduration ratio could be a critical factor in event selection.
By understanding these insights, we can approach the problem more effectively, using suitable data structures and algorithms to solve it.
Problem Boundary
The scope of this problem involves:
Understanding the Problem: First, it’s crucial to grasp the conditions set forth in the problem. This includes understanding that you’re given a list of events, each with a start and end day and a value, as well as a maximum number of events you can attend. You’re required to find the maximum possible sum of values from the events you can attend, with the constraint that no two events overlap on any day.
Developing an Algorithm: The solution involves developing an algorithm that can efficiently select the optimal set of events to maximize the sum of values while respecting the constraints on overlapping events and the maximum number of events. This typically involves leveraging data structures and sorting, greedy algorithms, or dynamic programming.
Implementing the Solution: This involves writing a function or program that accepts the inputs in the form specified, applies the algorithm to find the best combination of events to attend, and returns the maximum sum of event values.
Optimizing the Solution: Given the constraints in the problem statement, it’s important to design an algorithm that is not only correct but also efficient in terms of time and space complexity.
Testing the Solution: Lastly, the scope includes testing the solution with a variety of test cases, including edge cases, to ensure it works correctly in all scenarios.
Therefore, the scope spans from understanding the problem and creating an algorithm, to coding an optimized solution and testing its correctness and efficiency.
Establishing the boundary of a problem involves clearly defining the inputs, outputs, and constraints, as well as the limits within which the problem must be solved. For this particular problem, the boundaries can be established as follows:
Inputs: The inputs are an array of events and an integer
k
. Each event is represented by an array of three integers,[startDay, endDay, value]
, wherestartDay
andendDay
represent the start and end days of the event, andvalue
is the value obtained by attending the event. The integerk
is the maximum number of events you can attend.Outputs: The output is a single integer representing the maximum sum of values you can achieve by attending the events, considering that no two events can overlap and you can attend at most
k
events.Constraints: The problem specifies a number of constraints, including limits on the number of events, the range of start and end days, and the values of the events. The events cannot overlap, and the total number of events you can attend is capped at
k
.Functionality: The solution must accurately calculate the maximum sum of values from the events you can attend, adhering to the constraints of the problem. It’s not enough just to select the events with the highest values, as this may violate the constraint about overlapping events.
Performance: The solution should be efficient in terms of both time and space complexity. Given the constraints on the size of the input, an efficient algorithm is necessary to solve the problem within a reasonable time.
By establishing these boundaries, we create a clear understanding of what the problem is asking for, what the output should look like, and under what conditions the problem should be solved.
Distilling the Problem to Its Core Elements
Fundamental Concept: The fundamental concept in this problem is greedy algorithms and dynamic programming. A greedy approach is where we make the locally optimal choice at each stage with the hope of finding a global optimum. Dynamic programming is a method for solving problems by breaking them down into smaller, simpler subproblems and using the solutions to those subproblems to build up the solutions to larger ones.
Simple Description: Imagine you have a list of special events each offering a certain reward for attending. However, each event lasts for a few days and you can only attend one event at a time. Also, you have a limit on the total number of events you can attend. Your task is to figure out which events to attend to gain the maximum total reward.
Core Problem: The core problem is to maximize the total reward by choosing which events to attend, given that no two events can overlap and there is a limit on the total number of events that can be attended.
Key Components: The key components are:
 The start and end days of each event
 The reward for attending each event
 The maximum number of events that can be attended.
Minimal Set of Operations:
 Sorting the events based on their end days
 Iterating through the events and for each event, considering whether to attend it or skip it
 If attending the event, updating the maximum total reward and keeping track of the number of events attended
 Ensuring that the total number of events attended does not exceed the given limit and that no two events overlap.
This problem is essentially about making the most strategic choices based on the given constraints, and it involves concepts of decision making, optimization, and planning under constraints.
Simple Explanation
Imagine you are a very popular person and have been invited to multiple birthday parties. Now, these aren’t ordinary parties; at each party, you get a gift bag with a certain number of candies. But there are a few rules:
 You can only attend one party at a time, and if you decide to go to a party, you have to stay for the whole time, which might be more than one day.
 You can only attend a certain maximum number of parties because of your busy schedule.
 Some parties might be happening at the same time, and since you can only attend one at a time, you have to choose wisely.
 Your goal is to get as many candies as possible.
Now, you’re sitting with your calendar and a list of parties, each with its start date, end date, and the number of candies you’ll get. You need to figure out the best way to choose which parties to attend to maximize your total candy haul.
That’s pretty much what this problem is about, except replace “parties” with “events”, “days” with “time periods”, and “candies” with “values”. And of course, we’re asking a computer to do the scheduling, not a person.
Case Analysis
Example 1:
Input: events = [[1,2,4], [3,4,3], [2,3,10]], k = 2
Output: 14
Explanation: The best strategy here is to attend events 0 and 2. Event 0 runs from day 1 to 2 and gives you 4 candies. Event 2 runs from day 2 to 3 and gives you 10 candies. So, the total is 14 candies. Even though you could attend two events, you should not attend event 1 because it overlaps with event 2 which has a higher value.
Example 2:
Input: events = [[1,3,5], [2,5,6], [6,9,7]], k = 1
Output: 7
Explanation: Here, you can only attend one event. Even though the first two events have overlapping periods, you should attend the third event which doesn’t overlap with the others and gives you the maximum candies.
Example 3:
Input: events = [[1,3,5], [4,6,7], [7,9,6]], k = 3
Output: 18
Explanation: Here, you can attend all the events because none of them overlap with each other. So you get a total of 5 + 7 + 6 = 18 candies.
Example 4: (Edge Case)
Input: events = [[1,1,5]], k = 1
Output: 5
Explanation: There is only one event to attend, and you can attend up to one event. So you attend it and get 5 candies.
Example 5: (Boundary Case)
Input: events = [[1,1000000000,5]], k = 1
Output: 5
Explanation: There’s just one event here, but it lasts a very long time (from day 1 to day 1000000000). However, since it doesn’t overlap with any other event (because there are no other events), you can attend this one.
Analyzing these examples, we can understand that the problem requires careful selection of events to attend. The value of the event (candies in the analogy), the duration of the event, and the number of events that can be attended are all important factors. The solution needs to account for overlaps and choose the optimal combination of events to attend.
Identification of Applicable Theoretical Concepts
There are a few algorithmic concepts that can help in solving this problem more efficiently.
Sorting: By sorting the events based on their end day, you can make sure that you are always considering events with the earliest end day first. This way, you can optimize your choices and leave room for other potential events to attend.
Priority Queue (Heap): A priority queue can be used to store the events that you have attended, ordered by their values. This will allow you to quickly access and possibly remove the event with the lowest value in case you encounter a new event with a higher value and you have already attended the maximum allowed number of events (k).
Dynamic Programming: This problem can also be thought of as a variant of the classic Knapsack problem, which is usually solved by dynamic programming. You can create a DP table where the state DP[i][j] represents the maximum value you can achieve by attending up to ‘i’ events and using ‘j’ slots. However, given the large constraints of the problem, this approach would require careful optimization to avoid high time complexity.
Binary Search: This concept might be less directly applicable, but in case you are sorting the events and you want to quickly find out for a given event which is the previous event that doesn’t overlap with it, binary search could be a possible way to achieve this. However, the applicability of binary search would depend on the exact approach you are taking to solve the problem.
These are general algorithmic concepts that could be applied to this problem. The exact application and effectiveness of these concepts would depend on how you decide to approach the problem.
Problem Breakdown and Solution Methodology
Let’s break down the problemsolving process into steps. We’ll use an analogy to make it more intuitive.
Imagine that you are an event planner with a quota to meet  you can only plan ‘k’ events. You have a list of potential events, each with a start and end date, and a value  let’s say this is the popularity of the event. Your goal is to plan the events in such a way that you maximize the total popularity.
Here’s how we can approach this:
Sort the events: Start by sorting the events based on their end dates. If two events have the same end date, sort them based on their start dates. The rationale behind this is to give priority to events that end earlier, leaving room for potentially more events later.
Iterate through the events: Go through the sorted list of events. For each event, check if it’s possible to attend this event  that is, it doesn’t overlap with an event you’ve already planned.
If you can attend the event, add it to your plan (we will use a max heap to keep track of these). The heap will be sorted in ascending order based on event values. This way, the event with the least popularity will be at the top of the heap.
If you cannot attend the event because it overlaps with another one you’re already planning, check if this event has a higher value than the least popular event you’re planning (which is the one on top of the heap). If it does, replace that event with the current one in the heap.
Check the quota: At any point, if the size of your heap (the number of events you’re planning) exceeds ‘k’, remove the least popular event from your plan (the one at the top of the heap). This makes sure that you are always planning the ‘k’ most popular events that do not overlap with each other.
Calculate the total popularity: Once you have iterated through all the events, the ones left in the heap are the ones you should plan. Their total popularity (the sum of their values) will be the maximum value you can achieve.
To understand how this works, let’s consider an example:
Suppose you have the following events (start date, end date, popularity): [[1,2,4], [3,4,3], [2,3,10]], and you can plan 2 events.
 After sorting the events, you get: [[1,2,4], [2,3,10], [3,4,3]].
 You start by planning the first event: [[1,2,4]]. The total popularity is 4.
 The next event [2,3,10] overlaps with the one you’re planning, but it has a higher popularity. So you replace [1,2,4] with [2,3,10] in your plan. The total popularity is 10.
 The last event [3,4,3] doesn’t overlap with any event you’re planning, so you add it to your plan: [[2,3,10], [3,4,3]]. The total popularity is 13.
 Since you can only plan 2 events, you remove the least popular one from your plan: [[2,3,10]]. The maximum total popularity you can achieve is 10.
In terms of time complexity, the sorting operation is O(n log n) and each insert/delete operation on the heap is O(log k), where ’n’ is the number of events and ‘k’ is the maximum number of events you can attend. Therefore, the overall time complexity of this approach is O(n log n + n log k).
Inference of ProblemSolving Approach from the Problem Statement
Here are some key terms and concepts and how they inform the approach to solve this problem:
Events with a start date, end date, and value: This aspect of the problem suggests that each event can be represented as a distinct entity with specific properties. We can use a data structure, like a list of lists or a list of tuples, to represent these events. This also allows us to easily sort the events based on their properties.
Maximum number of events (k) to attend: This constraint indicates that we need a way to keep track of the events we choose to attend and limit their number. This suggests the use of a data structure like a heap to store these events and keep track of their total value.
Overlapping of events: Since two events cannot be attended if they overlap, we need to consider this when choosing which events to attend. The problem’s sorting strategy (sorting events based on end date, then start date) helps to ensure that we are always able to attend the maximum possible number of nonoverlapping events.
Maximizing the sum of values: This is the ultimate goal of the problem. As we iterate through the sorted list of events, we always aim to maximize the total value of the events we choose to attend. The use of a max heap helps in this regard because we can easily identify and replace the event with the lowest value if a higher value event becomes available.
For visualization, you could draw a timeline for the events and color the events you choose to attend. For instance:
Events: [[1,2,4], [3,4,3], [2,3,10]], k = 2
Sorted events: [[1,2,4], [2,3,10], [3,4,3]]
Timeline:
1 2 3 4 5

4 10 3
Attended events:
1 2 3 4 5

10 3
Max value: 13
This visual representation can help you understand the sequence of events and how the algorithm selects which events to attend.
How did you infer from the problem statement that this problem can be solved using ?
Simple Explanation of the Proof
I’m having trouble understanding the proof of this algorithm. Could you explain it in a way that’s easy to understand?
Identify Invariant
An invariant, in the context of programming or problem solving, is a condition or property that remains unchanged throughout the execution of an algorithm or throughout the problemsolving process.
In this problem, one possible invariant is the following:
At each step in the algorithm, the priority queue (or max heap) always contains the events that have the highest values and can be attended so far, but no more than ‘k’ events at any point.
This invariant is maintained throughout the execution of the algorithm:
At the start of the algorithm, the heap is empty which adheres to this invariant as there are no events that can be attended “so far”.
When considering each event, we only add it to the heap if it doesn’t overlap with the event that finishes last in the heap, or if it has a higher value than the least valuable event in the heap and can replace it. This maintains our invariant that the heap contains the highest value, nonoverlapping events that can be attended so far.
If adding an event causes the heap size to exceed ‘k’, we immediately remove the least valuable event from the heap, maintaining the limit of ‘k’ events.
By the end of the algorithm, we have considered all events, and the heap contains the ‘k’ highestvalue events that we can attend without any overlaps.
Thus, the invariant guides our algorithm design and helps ensure the correctness of our solution.
Identify Loop Invariant
A loop invariant is a condition that is initially true and remains true after each iteration of a loop.
In this problem, one possible loop invariant related to the algorithm that solves it could be:
“At the start of each iteration of the main sorting loop, all events in the array are sorted by their endDay in nondecreasing order up to the current index, and all of the highestvalue, nonoverlapping events that can be attended so far (up to ‘k’ events) are stored in the priority queue (or heap).”
This loop invariant holds true under the following conditions:
Initialization: Before the first iteration (index 0), no events have been processed yet, so the ‘sorted up to the current index’ condition is trivially true, and the priority queue is empty, which fits the ‘highestvalue, nonoverlapping events’ condition.
Maintenance: During each iteration, we process the next event in the sorted array. If this event can be attended without overlapping with the last event in the heap, or if it has a higher value than the least valuable event in the heap and can replace it, we add it to the heap. If this makes the heap size exceed ‘k’, we remove the least valuable event. This maintains our invariant.
Termination: At the end of the loop, all events have been processed, and our heap contains the ‘k’ highestvalue, nonoverlapping events that we can attend, as desired.
This loop invariant, therefore, helps ensure the correctness of our algorithm and guides the design of our solution.
Stepwise Refinement
 Stepwise Refinement
Here’s how we can refine our approach:
 Step 1: Parse the Input  Understand and validate the input format.
 Step 2: Sort the Events  Sort the events based on their ending day. If two events end on the same day, the one with the earlier starting day comes first. This ensures that if we have to choose between two events with the same end day, we always choose the one with the earlier start day, leaving more days available for future events.
 Step 3: Create a Max Heap  Implement a max heap to keep track of the events we have decided to attend. The max heap allows us to quickly determine the event with the lowest value if we need to replace it.
 Step 4: Iterate Over the Events  For each event in the sorted list, check if it can be attended (i.e., if its start day is later than the end day of the last attended event). If it can, add it to the heap.
 Step 5: Maintain Heap Size  After adding an event to the heap, if the size of the heap exceeds ‘k’ (the maximum number of events we can attend), remove the event with the lowest value. This ensures that we always attend the ‘k’ highestvalue events.
 Granular, Actionable Steps
From our stepwise refinement, we can further break down the problem into more granular, actionable steps:
 Parse the input to obtain a list of events.
 Implement a comparison function that sorts events based on their ending day, and if two events end on the same day, the one with the earlier starting day comes first.
 Use a priority queue to implement a max heap.
 Write a loop to iterate over the sorted list of events. For each event, check if its start day is later than the end day of the last attended event. If it is, add it to the heap.
 After adding an event to the heap, if the size of the heap exceeds ‘k’, remove the event with the lowest value from the heap.
 Independent Parts
The sorting of the events, the creation and management of the max heap, and the main loop that iterates over the events can all be implemented independently of each other.
 Repeatable Patterns
One repeatable pattern is the main loop where we iterate over the sorted list of events, decide whether to add them to our heap, and if necessary, remove an event from the heap. The process of managing the heap (adding an event if possible, ensuring its size doesn’t exceed ‘k’, and removing the lowest value event if needed) is a repeatable operation that we perform for each event in our sorted list.
Solution Approach and Analysis
Let’s first restate the problem in a simpler way: You have a bunch of events that you can go to, each event gives you a certain value, but you can only go to a limited number of them. Your job is to pick the events to maximize your total value. However, you cannot attend two events at the same time, so you must take into consideration the start and end dates of the events.
Here’s how we could tackle this problem, breaking down the process into steps:
 Step 1: Sort the Events
The first step would be to sort the events based on their end day in ascending order. If two events end on the same day, sort them based on their start day, so the one that starts earlier comes first. This sorting is necessary to manage overlapping events.
For instance, if you think of these events as shows on TV, you would arrange your schedule based on the time the shows end, so you can tune into the next show immediately after. If two shows end at the same time, you would prefer to watch the one that starts earlier.
 Step 2: Set Up a Max Heap
Next, set up a max heap (or priority queue) that will store the events you’ve chosen to attend. The max heap will be ordered by the value of the events, so the event with the smallest value is always on the top. Think of this like a bag that always keeps the least valuable item at the top, so it’s easy to remove if you find something more valuable.
 Step 3: Iterate Over the Events
Now, iterate over your sorted list of events. For each event, check if you can attend it, which would mean its start day is after the end day of the last event in your heap (or your last watched TV show ended before this one starts).
 Step 4: Manage Your Heap
If you can attend the current event, add it to your heap. If adding this event causes the size of the heap to exceed ‘k’, the maximum number of events you can attend, then remove the smallest event from your heap (or remove the least valuable item from your bag to make room for a more valuable one).
 Step 5: Return Your Total Value
Once you’ve gone through all the events, the total value of events in your heap will be your maximum achievable value. Return this value.
Now, let’s discuss how specific changes in the problem’s parameters would affect the solution:
 If ‘k’ increases, the maximum total value will either stay the same or increase. This is because you can attend more events.
 If the start and end days of an event overlap more with other events, it becomes less likely to be chosen unless its value is much higher than the others.
 If the values of the events increase, the maximum total value will increase.
Lastly, let’s demonstrate this approach using an example:
Suppose we have events = [[1,2,4],[3,4,3],[2,3,10]], k = 2. Here are the steps we would follow:
 After sorting the events, we have [[1,2,4],[2,3,10],[3,4,3]].
 We start with the first event [1,2,4] and add it to our heap.
 The next event is [2,3,10], but its start day is not later than the end day of our current event in the heap. However, 10 > 4, so we replace [1,2,4] with [2,3,10] in our heap.
 The final event is [3,4,3], and its start day is later than the end day of our current event in the heap, so we add it.
 Now our heap is [[2,3,10],[3,4,3]], and the total value is 13. So the maximum value we can achieve by attending events is 13.
Thought Process
The problem demands a good understanding of sorting, greedy algorithms, and data structures like heap. Let’s break down the problem and try to formulate a plan for our code.
The main idea is that we want to attend the events that have the most significant value, but the constraint of being able to attend only one event at a time means we need to choose carefully which events to attend, especially when they overlap.
Step 1: Sort the events We start by sorting the events based on their end day. The rationale behind this is that if two events have the same start day, we would rather choose the one that ends earlier because it keeps us available for more events.
Step 2: Use a heap to store events We use a max heap, where the key is the negative of the event’s value. This way, the event with the smallest value will be at the top of the heap. We choose a max heap because it efficiently supports the operations we need: inserting events, finding the event with the smallest value, and removing that event if needed.
Step 3: Iterate over the events Now, we iterate over the sorted list of events. For each event, we add it to our heap. Then, while the size of our heap is greater than the number of events we can still attend, we remove the top event from our heap. This ensures that we are always ready to drop the event with the least value if we find a more valuable event.
Step 4: Sum the values After going through all the events, our heap will contain the ‘k’ most valuable nonoverlapping events that we can attend. The sum of their values is the maximum sum we can achieve.
Let’s implement this in Python3:


Note that this code assumes that ’events’ is a list of lists, where each inner list has three elements: the start day, end day, and value of each event. Also, it assumes that ‘k’ is an integer that represents the maximum number of events you can attend.
Buggy Solution.
Hints:
Sort the events by its startTime. For every event, you can either choose it and consider the next event available, or you can ignore it. You can efficiently find the next event that is available using binary search.


From Brute Force to Optimal Solution
Let’s start with a brute force approach.
Brute Force Solution
The simplest but least efficient solution would be to generate all possible combinations of events that we can attend, calculate the total value of each combination, and return the maximum. This approach involves iterating through all the subsets of events, which will be a time complexity of O(2^n), where ’n’ is the number of events. For each subset, we would also need to check if the events overlap, which adds another O(n) time complexity factor. This makes the overall time complexity O(n * 2^n), which is highly inefficient and will not run in a reasonable time when the number of events is large.
In Python, we could implement this as follows:


Optimized Solution
To optimize this solution, we could employ a greedy approach, coupled with sorting and a priority queue (heap).
As I explained earlier, we can sort the events by their ending day. This way, we always have the option to choose the event that ends earlier when two events start on the same day. A min heap will be used to keep track of the events we plan to attend. We only add an event to the heap if attending it doesn’t make us exceed ‘k’ events or we can replace a less valuable event in the heap.
The optimized solution has a time complexity of O(n log n) for sorting the events and O(n log k) for managing the heap, giving an overall time complexity of O(n log n + n log k), which is a significant improvement over the brute force solution.
The space complexity is O(n) for storing the sorted events and O(k) for the heap, resulting in an overall space complexity of O(n + k).
Here’s what the Python code might look like:


This code offers the same functionality as the brute force solution but is significantly more efficient. It solves the problem in a reasonable amount of time even when the number of events is large.
Establishing Preconditions and Postconditions
Parameters:
 The method takes two parameters:
events
andk
. events
is a list of lists where each sublist is an event defined by three integers:startDayi
,endDayi
,valuei
.k
is an integer.events
represents the array of events where each event is characterized by its start day, end day, and value.k
represents the maximum number of events that can be attended.
 The method takes two parameters:
Preconditions:
 Before this method is called, it must be ensured that
events
is a nonempty list andk
is a positive integer.  The constraints on the input parameters are specified in the problem statement: 1 <= k <= events.length, 1 <= k * events.length <= 10^6, 1 <= startDayi <= endDayi <= 10^9, and 1 <= valuei <= 10^6.
 There’s no specific state that the program or some part of it must be in before the method is called.
 Before this method is called, it must be ensured that
Method Functionality:
 This method is expected to find the maximum sum of values that can be received by attending a maximum of
k
events. It does so by sorting the events by their end day, and then using a heap to keep track of the events to attend.  It doesn’t interact with the current state of the program, only with the inputs.
 This method is expected to find the maximum sum of values that can be received by attending a maximum of
Postconditions:
 After the method has been called and has returned, the state of the program or the values of the parameters remain unchanged. The method is purely functional and doesn’t have any side effects.
 The return value represents the maximum sum of values that can be received by attending a maximum of
k
events.  There are no side effects.
Error Handling:
 The method doesn’t explicitly handle errors. If the preconditions are not met, a Python exception might be thrown (for example, if
events
is not a list ork
is not an integer).  It doesn’t return a special value in case of an error. It’s expected that the preconditions are met when the method is called.
 The method doesn’t explicitly handle errors. If the preconditions are not met, a Python exception might be thrown (for example, if
Problem Decomposition
Problem Understanding:
 This problem asks to calculate the maximum value that can be achieved from attending up to k events. Each event has a start day, an end day, and a value. You can only attend one event at a time, and if you choose an event, you have to attend it for the full duration. You cannot attend two events that overlap on any day.
Initial Breakdown:
 The broad subproblems here are:
 Identifying which events can be attended without overlapping
 Calculating the maximum possible value from attending up to k events
 The broad subproblems here are:
Subproblem Refinement:
 For the first subproblem, it involves sorting the events by their start day and using a binary search to find the next nonoverlapping event.
 For the second subproblem, it involves maintaining a dynamic programming table to store the maximum value that can be achieved by attending a certain number of events.
Task Identification:
 The repeated tasks are:
 Determining if a new event can be attended based on the end day of the current event
 Updating the DP table with the maximum value after considering each new event
 The repeated tasks are:
Task Abstraction:
 These tasks are abstract enough to be clear and reusable within the context of the problem.
Method Naming:
 The binary search task can be named as
binary_search()
 The main task of calculating the maximum value can be named as
maxValue()
 The binary search task can be named as
Subproblem Interactions:
 The tasks must be performed in a specific order. First, the events are sorted by their start day. Then, for each event, we perform a binary search to find the next nonoverlapping event and update the DP table. The interactions between the tasks are that the result of the binary search influences how we update the DP table.
Coding Constructs
Consider the following piece of complex software code.
What are the highlevel problemsolving strategies or techniques being used by this code?
If you had to explain the purpose of this code to a nonprogrammer, what would you say?
Can you identify the logical elements or constructs used in this code, independent of any programming language?
Could you describe the algorithmic approach used by this code in plain English?
What are the key steps or operations this code is performing on the input data, and why?
Can you identify the algorithmic patterns or strategies used by this code, irrespective of the specific programming language syntax?
Code Explanation and Design Decisions
Initial Parameters:
 The initial parameters are the list of events and the integer k. The list of events, each represented by a start day, an end day, and a value, provides the information about all available events. The integer k represents the maximum number of events that can be attended.
Primary Loop or Iteration:
 The primary loop is over the list of events. Each iteration represents the consideration of a new event that could be potentially attended. The iteration advances the solution by adding to the potential value that could be gained from attending the events.
Conditions or Branches:
 Within the loop, there is a condition that checks whether the new event could be attended without overlapping with the current event. This condition signifies the constraint that no two events can be attended at the same time. The logic behind this condition is that if the start day of the new event is after the end day of the current event, then the new event can be attended.
Updates or Modifications:
 The dynamic programming (DP) table is updated within the loop to reflect the maximum value that can be achieved by attending a certain number of events. This update is necessary to keep track of the optimal solution so far. These modifications reflect the new possibilities of attending events and increasing the total value.
Invariant:
 The invariant in the code is that the DP table always contains the maximum value that can be achieved by attending a certain number of events. This invariant helps to ensure that the final solution is the optimal one, satisfying the problem’s objective of maximizing the total value.
Final Output:
 The final output is the maximum value that can be achieved from attending up to k events. This output satisfies the problem’s requirements as it represents the maximum value that can be achieved while satisfying all constraints, such as not attending overlapping events and not attending more than k events.
Language Agnostic Coding Drills
Identification of Coding Concepts:
a. Variable Declaration and Initialization: This is the most basic concept which is about understanding how to declare and initialize variables.
b. Array/Lists Manipulation: This is about understanding how to create, access, and modify elements in an array or list.
c. Sorting: This is about understanding how to sort an array or a list based on one or multiple conditions.
d. Binary Search: This is an algorithm used to find a particular element in a sorted list.
e. Looping Constructs: This is about understanding how to use loops to iterate over arrays or lists.
f. Conditional Statements: This involves understanding how to use conditions to guide the logic flow of the program.
g. Dynamic Programming: This is an advanced concept which involves solving a problem by breaking it down into simpler, smaller subproblems and storing the results of these subproblems to avoid duplicate work.
Difficulty Level:
a. Variable Declaration and Initialization: Easy  This is the most basic concept that every coder learns first.
b. Array/Lists Manipulation: Easy  This is a fundamental concept which is widely used and relatively simple to understand.
c. Sorting: Intermediate  While sorting itself is a builtin function in many languages, understanding when and how to apply it requires a deeper understanding.
d. Binary Search: Intermediate  Binary Search is a simple yet powerful algorithm but understanding and implementing it requires knowledge about data structures and algorithms.
e. Looping Constructs: Easy  Loops are a basic control structure in programming and are generally one of the first concepts learned.
f. Conditional Statements: Easy  Conditionals guide the logical flow of a program and are one of the first constructs learned.
g. Dynamic Programming: Hard  Dynamic programming involves more complex logic and requires the ability to break a problem down into smaller subproblems, which is not straightforward for beginners.
ProblemSolving Approach:
The problem at hand can be solved using a combination of the abovementioned coding concepts.
Start by declaring and initializing necessary variables and data structures, including the array or list that will be manipulated.
Apply the concept of Array/List Manipulation to create a list of events, where each event is represented by a start time, an end time, and a value.
Use the Sorting concept to sort the events based on start time. This is a crucial step because the order of events can impact the final result.
Next, use the concept of Looping Constructs to iterate over the sorted list of events. For each event, use Conditional Statements to check if it can be attended without overlapping with the previous event.
Within this loop, use Binary Search to find the next available event that doesn’t overlap with the current event.
Finally, apply the concept of Dynamic Programming. Maintain a dynamic programming table that records the maximum value that can be obtained by attending a certain number of events. Update this table as you iterate through the events.
The final output is the maximum value that can be achieved from attending up to a certain number of events. This output satisfies the problem’s requirements as it represents the optimal solution.
Targeted Drills in Python
Coding Drills for General Concepts:
a. **Variable Declaration and Initialization:**
```python
num = 0 # declare an integer
string = "" # declare a string
array = [] # declare a list
dictionary = {} # declare a dictionary
```
b. **Array/Lists Manipulation:**
```python
array = [1, 2, 3, 4, 5] # array declaration
array.append(6) # add an element
array.remove(1) # remove an element
length = len(array) # get the length of array
print(array[0]) # access an element
```
c. **Sorting:**
```python
array = [3, 1, 2, 5, 4]
array.sort() # sort in ascending order
array.sort(reverse=True) # sort in descending order
```
d. **Binary Search:**
```python
from bisect import bisect_left
array = [1, 3, 4, 4, 6, 7]
print(bisect_left(array, 4)) # find the leftmost index where 4 can be inserted to maintain sorted order
```
e. **Looping Constructs:**
```python
array = [1, 2, 3, 4, 5]
for num in array: # iterate over an array
print(num)
```
f. **Conditional Statements:**
```python
num = 5
if num > 0: # if statement
print("Positive")
elif num == 0: # elseif statement
print("Zero")
else: # else statement
print("Negative")
```
Coding Drills for ProblemSpecific Concepts:
g. **Dynamic Programming:**
```python
def fibonacci(n):
dp = [0, 1] + [0]*(n1) # declare a dp table with base cases
for i in range(2, n+1): # loop from base cases to n
dp[i] = dp[i1] + dp[i2] # transition formula
return dp[n] # return the final result
print(fibonacci(10)) # 55
```
Dynamic programming is an essential part of this problem as it allows us to optimize our solution by reusing previously computed results, thus saving computation time. In this case, we use a dynamic programming approach to store and reuse the maximum value we can get by attending a certain number of events, which directly contributes to our final solution.
Integration:
We would begin by declaring and initializing necessary variables (Concept a). Next, we would apply the concept of Array/List Manipulation to create a list of events (Concept b).
We would then use the Sorting concept to sort the events based on their starting time (Concept c). After sorting, we would iterate over these events using the Looping Constructs (Concept e).
For each event, we would use Conditional Statements to check whether it can be attended without overlapping with the previous event (Concept f). If it can, we would use Binary Search to find the next available event that doesn't overlap with the current event (Concept d).
Finally, we would apply the Dynamic Programming concept (Concept g) to keep track of the maximum value we can get by attending a certain number of events. As we iterate over the events, we update this table.
The final output is the maximum value that can be achieved from attending up to 'k' number of events. This output satisfies the problem's requirements as it represents the optimal solution.
Q&A
Similar Problems
Here are 10 LeetCode problems that require similar problemsolving strategies or use similar underlying concepts as the problem we’ve just solved. They involve similar steps or techniques like sorting, binary search, dynamic programming, and handling intervalbased problems.
Problem LeetCode 56: Merge Intervals: This problem requires sorting intervals by their start time and then merging overlapping intervals, similar to how we had to sort events by their start time and ensure that we only attend nonoverlapping events.
Problem LeetCode 646: Maximum Length of Pair Chain: This problem is about selecting a maximum number of pairs with the second number of the pair less than the first number of the next pair. This is similar to choosing events with nonoverlapping time.
Problem LeetCode 300: Longest Increasing Subsequence: This problem involves using binary search and dynamic programming to find the longest increasing subsequence, similar to how we used these techniques to find the maximum value from attending events.
Problem LeetCode 435: Nonoverlapping Intervals: This problem asks to eliminate overlapping intervals to leave as many nonoverlapping intervals as possible, which requires similar logic to choosing nonoverlapping events to attend.
Problem LeetCode 1235: Maximum Profit in Job Scheduling: This problem is very similar to the original one, as it involves selecting jobs (similar to events) with start and end times so as to maximize profit. It also requires sorting, binary search, and dynamic programming.
Problem LeetCode 452: Minimum Number of Arrows to Burst Balloons: This problem involves intervals and selecting minimum arrows to cover all intervals. The concept is similar to the event selection in the original problem.
Problem LeetCode 1105: Filling Bookcase Shelves: This problem is about arranging books on shelves to minimize the total height of the bookcase. It uses dynamic programming similar to our original problem.
Problem LeetCode 1092: Shortest Common Supersequence: This problem is about finding the shortest common supersequence of two strings. It involves dynamic programming to keep track of the results of subproblems, similar to the original problem.
Problem LeetCode 354: Russian Doll Envelopes: This problem is about selecting a maximum number of envelopes that can be put one inside another. It involves sorting and finding longest increasing subsequence, similar to our original problem.
Problem LeetCode 139: Word Break: This problem involves using dynamic programming to break a string into words from a given list, which is similar to our original problem where we used dynamic programming to maximize the value of attended events.