Prefix Sums
6 min · C# examples
Precompute running totals so any range sum becomes a quick subtraction.
C# · Prefix Sums0/5 cleared
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A prefix sum at index i stores the total of everything from the start up to i. Build it once, then the sum of any slice [L..R] is prefix[R] - prefix[L-1].
This turns repeated range-sum questions from O(n) each into O(1) each after O(n) setup.
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