This is Subarray Sum Equals K from LeetCode. It's one of the most common prefix sum interview questions. Given an array of integers and an integer , return the total number of subarrays whose sum equals .
At first, you might try checking every subarray. That's subarrays, and even with prefix sums, you're still doing comparisons. Can you do better? Before reading the solution, try to identify the base case and recursive relationship yourself.