LeetCode Card 228 — Prefix Sums / Structure
Pattern signal
This pattern usually appears when the prompt involves range sums, subarray totals, balance counting, or repeated aggregate queries. Subarray Sum Equals K, Range Sum Query, and running-balance problems usually point here.
What Structure means here
In BASIC, the Structure step is where you state the invariant, helper contract, or control-flow skeleton before coding. For Prefix Sums, that matters because the pattern only becomes useful once the candidate is explicit about what is being tracked, reduced, or preserved.
Interview move
A strong move is to make the plan visible before full execution. In this pattern family, say out loud what representation makes the problem easier: the map entry, the pointer invariant, the recursion contract, the queue contents, or the DP state. That keeps the implementation attached to a reason.
Common miss
Candidates often recompute totals repeatedly instead of carrying forward cumulative state. BASIC reduces that risk because the stage sequence forces you to earn the implementation instead of jumping straight into it.
BASIC prompt
“Given that this looks like Prefix Sums, what is the simplest way to state the invariant, helper contract, or control-flow skeleton before coding?”