LeetCode Card 226 — Prefix Sums / Breakdown
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 Breakdown means here
In BASIC, the Breakdown step is where you identify the signal words, constraints, and output shape that reveal the pattern family. 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 split the problem before trying to solve it. 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 identify the signal words, constraints, and output shape that reveal the pattern family?”