What is Prefix Sum

prefix sum (cumulative sum array) precomputes cumulative sums so any subarray sum can be calculated in $O(1)$. You use it when answering multiple range sum queries or finding subarrays with target sums. It trades $O(n)$ preprocessing for $O(1)$ queries, transforming $O(n)$ per query to $O(1)$.

You'll see this in range queries, subarray sums, and matrix problems. If you can explain these concepts in your own words, you'll solve related problems faster under interview pressure.