Subarray Sum Equals K

The problem. Given an array of integers nums and an integer k, return the number of subarrays whose elements add up to exactly k. The numbers may be negative.

Example

nums = [1, 1, 1],                   k = 2  ->  2    ([1, 1] twice)
nums = [1, 2, 3],                   k = 3  ->  2    ([1, 2] and [3])
nums = [3, 4, 7, 2, -3, 1, 4, 2],   k = 7  ->  4