Decrease and Conquer at Five Levels
Level 1: Child
Let’s say you have a big stack of blocks, and you want to move the whole stack from one place to another. But the rule is you can only move one block at a time. So, you start by moving the top block, then the next one, and so on. You are using a “Decrease and Conquer” approach. You take a big job, move one block, and make the job a little bit smaller each time until there are no blocks left to move.
Level 2: Teenager
Let’s say you’re playing a video game, and you have a big boss to defeat. Instead of trying to beat the boss all at once, you aim to reduce its energy bit by bit until it’s defeated. This is a strategy called “Decrease and Conquer.” In computer science, we use this method to solve complex problems by breaking them down into smaller and more manageable parts.
Level 3: Undergraduate
“Decrease and Conquer” is an algorithmic design paradigm. Consider the problem of binary search, where you’re searching for a number in a sorted list. Instead of checking every number, you look at the middle one first. If the number you’re looking for is higher, you repeat the process on the right half; if it’s lower, you look on the left half. By doing this, you decrease the problem size by half every time until you find the number.
Level 4: Grad Student
In “Decrease and Conquer,” we break down a problem into smaller instances and solve each individually. It’s more nuanced than divideandconquer, where problems are split into independent, roughly equal parts. In Decrease and Conquer, problems are usually reduced into a single smaller problem, like in insertion sort where you sort a single element at a time. It’s used in various algorithms and data structures and is a critical part of understanding computational complexity.
Level 5: Colleague
“Decrease and Conquer” represents a class of algorithms that solve a problem by reducing it to a smaller instance of the same problem, plus some simple computation. Unlike “Divide and Conquer,” where we divide the problem into roughly equal parts, Decrease and Conquer algorithms reduce the problem by a constant amount, such as in DepthFirst Search or the Euclidean algorithm for computing greatest common divisor. It’s a fundamental strategy for algorithm design and analysis, having widereaching implications in terms of efficiency and time complexity.
Recursion is indeed a common implementation method for “Decrease and Conquer” algorithms, but the concepts aren’t the same.
Socrates Teaches Decrease and Conquer
Have you ever had a large number of tasks to do, and you decided to handle them one at a time until none were left?
Right, we all have experienced such situations! Now, imagine you’re dealing with a problem in computer science or mathematics. Do you think a similar approach of tackling one piece at a time could be applicable?
Great! So, by solving one piece of the problem at a time, we gradually reduce the problem’s size. Can you guess what this problemsolving strategy is called in computer science and algorithms?
Close enough! We call this strategy “Decrease and Conquer”. It’s a method where we reduce the problem size by a constant, often by one, solve that smaller problem, and use that solution to solve the larger problem. This process is repeated until we solve the entire problem. It’s a useful way of handling certain kinds of problems in computer science, don’t you think?
Richard Feynman Explanation
Alright, let’s imagine you’re trying to sort a big pile of marbles. You’re in your backyard, it’s a sunny day, and you’ve got this huge mountain of different colored marbles in front of you. You want to sort them by color  all the reds here, all the blues there, and so on. But the pile is so big that it’s overwhelming.
So what do you do? You could try to sort all the marbles at once, but that seems like a daunting task. Instead, you start by taking a handful of marbles out of the pile. Now, you have a smaller task to focus on. You sort these marbles by color, and then you go back for another handful from the big pile. Little by little, you’re making progress.
This is the essence of the decrease and conquer approach in computer science. Instead of tackling a big problem all at once, you break it down into smaller, manageable parts. You solve one part, and then move on to the next. This approach is used in many algorithms in computer science, including some sorting algorithms and search algorithms. It’s a simple, yet powerful technique that helps make complex problems easier to solve.
A Detailed Look at Problem Solving with Examples
Decrease and conquer is a powerful problemsolving strategy. The method works by breaking down a problem into a smaller instance of the same problem, then using the relationship between the smaller problem and the larger one to find a solution. This method can be implemented in a topdown (recursively) or bottomup (iteratively) manner.
Decrease by a Constant: In this approach, the problem size decreases by a constant amount at each iteration, often by one. This constant can vary depending on the nature of the problem.
Example: Consider the algorithm for Insertion Sort. We start with the second element of the array and compare it with the first one, if it is smaller, we insert it in the correct position, that is in front of the first element. Then we take the third element and insert it in the correct position among the first two and so on. Each step involves decreasing the unsorted part of the array by one, until we’ve sorted the entire array.
Another example is Topological Sorting of a directed graph. We start by picking a node with no incoming edges (a “source” node), add it to our sorted list, and then remove it from the graph, decreasing the number of nodes to sort by one. We repeat this process until all nodes are sorted.
Decrease by a Constant Factor: Here, the problem size is reduced by a constant factor at each iteration. More often than not, this factor is two, resulting in the problem size being halved at each step.
Example: Binary Search is a classic example of decrease by a constant factor. We start by comparing the target value with the middle element of the array. If the target value is the same, we’ve found our answer. If it’s less, we repeat the process on the left half of the array, and if it’s more, on the right half. With each comparison, we halve the size of our problem.
Variable Size Decrease: Unlike the previous two variations, here the size of the problem decreases variably from one iteration to another, which could depend on the problem itself or the intermediate results.
Example: The Euclidean algorithm for calculating the Greatest Common Divisor (GCD) uses a variable size decrease approach. We start with two numbers and compute their remainder when the larger number is divided by the smaller one. We then replace the larger number with the smaller number and the smaller number with the obtained remainder, and repeat the process until the remainder is zero. The size of the problem decreases unpredictably at each step.
These variations of decrease and conquer strategy form a robust arsenal for tackling a multitude of problems in computing, and understanding how they work is key to leveraging them effectively.
Claude Explanation
Decrease and conquer is a problem solving technique where the problem is reduced to a smaller version of the same problem, rather than decomposed into disjoint subproblems like in divide and conquer.
Key aspects:
 Reduce the original problem to a smaller instance
 Solve the smaller problem recursively
 Combine the solution to small problem into overall solution
 Reduction is done by decreasing size of input
For example, in binary search the search space is halved each time, reducing to a smaller version of searching in arrays. Dynamic programming also uses this approach.
Example in Python:


At each step the input size is decreased until the base case is reached. The solutions to smaller problems are combined to give the final solution.
Similar decrease and conquer solutions can be written in Java, C++ for problems like Fibonacci, binary search etc. where the problem can be reduced to a smaller version of itself.