Subarray Sum Equals K (Count)

Given an integer array and a target `k`, return the number of contiguous subarrays whose sum equals `k`. The exact interview signature, constraints, and examples remain unspecified. Requirements Given an integer array nums and a target k, count the contiguous subarrays whose sum equals k. Count every matching start-and-end pair, including overlapping subarrays. The exact function signature, numeric constraints, and return type are unspecified; clarify them before coding. Notes Maintain a running prefix sum and a frequency map of prefix sums already seen. For each current prefix p, add the stored count of p - k before incrementing the count of p. Seed the map with {0: 1} so a matching prefix beginning at index 0 is counted. The lookup-before-increment order also handles k = 0 without counting an empty subarray. The canonical implementation runs in expected O(n) time and O(n) auxiliary space. A variable-size sliding window is not sufficient when values may include negatives. Preparation Implement the prefix-sum frequency-map solution from a blank editor and explain why the lookup happens before the current prefix is inserted. Dry-run an input with negative values, an all-zero input, and an input where multiple overlapping subarrays hit the same target. Compare the linear scan with an O(n^2) prefix-sum baseline and state what information the map compresses.