Prefix Sum: When to Use It

Start with the idea

A problem is a good match for Prefix Sum when it asks for immutable range sums, cumulative totals, balance points. Here the concrete task is: Return the sum from inclusive index 1 through 3. The visible input [5, 2, 7, 1, 4] is an example, not a definition of the whole problem. Ask what operations the solution must support and what the input guarantees. Precomputation pays when many ranges are queried. If values change between queries, an updateable structure may be needed; for one small query, a direct walk is simpler. The right choice follows from those guarantees, not from a familiar noun in the prompt.

The decisive invariant is: Store the sum before every boundary so a range is one subtraction. Try to say why it applies to the task without quoting the implementation. Which fact becomes known after one update, and which possible answers remain? A range method summarizes intervals so the same information is not recomputed. A static prefix table and an updateable tree make different tradeoffs; define endpoint conventions first. If that claim cannot be maintained on a legal input, the pattern is wrong or one of its preconditions is missing. A counterexample is more useful than a vague warning: change one property of the sample, trace the first two steps, and identify the exact claim that fails.

Compare costs before committing. The advertised bound is O(n) build · O(1) query · O(n) space; identify the operation responsible for it and the extra memory it needs.