LeetCode Card 208 — Arrays and Hashing / Structure
Pattern signal
This pattern usually appears when the prompt involves lookup, duplicate detection, counting, grouping, or direct index-based traversal. Problems like Two Sum, Contains Duplicate, Valid Anagram, Group Anagrams, and Top K Frequent Elements live 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 Arrays and Hashing, 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 sort or nest loops before asking whether constant-time membership or counting would simplify the prompt. 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 Arrays and Hashing, what is the simplest way to state the invariant, helper contract, or control-flow skeleton before coding?”