Subarray Sum Equals K

Problem

Given an integer array nums and integer k, return the total number of contiguous subarrays whose sum equals k.

Example

nums = [1,1,1], k = 2
Output: 2

Constraints

• 1 ≤ nums.length ≤ 2 × 10^4
• -1000 ≤ nums[i] ≤ 1000

Approach

Prefix-sum + hash map of counts. Note negatives rule out the sliding window.