LeetCode Card 229 — Prefix Sums / Implement
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 Implement means here
In BASIC, the Implement step is where you write the code in the same order as the chosen skeleton so the logic stays stable. 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 transcribe the plan instead of improvising. 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 write the code in the same order as the chosen skeleton so the logic stays stable?”