HashMap

Summary

A key-value data structure with average O(1) lookup — the single most useful structure for turning brute-force O(n²) solutions into O(n) ones.

• 1Solve frequency-counting and lookup-based problems using hashmaps.
• 2Practice recognizing when a nested loop can be replaced by a hashmap lookup.
• 3Learn hashmap-based patterns: two-sum variants, grouping, deduplication.