Subarray Sum Equals K
Keep a running prefix sum and a hash map of how many times each prefix sum has appeared. For each new prefix, look for prefix minus k to count matching earlier starts.
Count how many contiguous subarrays sum to k.
Subarray Sum Equals K
Keep a running prefix sum and a hash map of how many times each prefix sum has appeared. For each new prefix, look for prefix minus k to count matching earlier starts.