Subarray Sum Equals K

LeetCode 560. Count subarrays whose sum equals K. Prefix-sum + hashmap is the canonical solution; appears as the second half of Meta phone screens paired with a heavier first problem. Requirements Given an integer array nums and integer k, return the number of contiguous subarrays whose sum equals k. Values may be negative; brute-force O(n²) and sliding-window do NOT work. Canonical solution: prefix-sum hash — keep a running prefix sum, count how often prefix - k has appeared. Examples nums = [1,1,1], k = 2 → 2. nums = [1,2,3], k = 3 → 2. With negatives: nums = [3,4,-7,1,2,-1], k = 0 → 4. Notes Initialize the hashmap with {0: 1} to count subarrays that start at index 0. Common bug: returning longest subarray instead of count (that's LC 325). Frequent pairing in Meta phone screens: appears as Problem 2 after a heavier Tag-list problem (Kth Largest, Subarray-K variations). Preparation Write the prefix-sum + hashmap solution in under 5 minutes. Drill LC 560 + LC 325 (max-length variant) + LC 974 (divisible-by-k variant) as a triplet. Be ready to explain why sliding-window fails when negatives are allowed.