From O(n²) to O(n): How Hashmaps Unlock Linear-Time Subarray Sums

A developer shares how they optimized a subarray sum problem from brute force O(n²) to O(n) using prefix sums and a hashmap. The key insight: storing prefix sums in a hashmap allows O(1) lookups to find subarrays summing to target k. The technique reduces runtime from hours to milliseconds on large inputs.