Which of the following is a common use case for recursion?

A common use case for recursion is when a problem can be broken down into smaller, similar problems. This is often seen in tasks such as:

1. Tree or Graph Traversal: In computer science, trees and graphs are often used to represent hierarchical or connected data. Recursion is used to traverse these data structures, visiting each node or vertex once.

2. Sorting Algorithms: Some sorting algorithms, like quicksort and mergesort, use recursion to break the list down into smaller pieces, sort those, and then combine them back together.

3. Calculating Factorials: The factorial of a number is the product of all positive integers less than or equal to that number. This can be calculated using recursion, with the base case being that the factorial of 1 is 1.

4. Fibonacci Sequence: The Fibonacci sequence is a series of numbers in which each number is the sum of the two preceding ones. This can be calculated using recursion, with the base case being that the first two numbers in the sequence are 0 and 1.

5. Divide and Conquer Algorithms: Recursion is often used in divide and conquer algorithms, where a problem is divided into smaller subproblems, each of which is solved individually. The solutions to the subproblems are then combined to give a solution to the original problem.