hash map

The canonical warm-up that screens for the hash-map instinct: trade space for time to turn O(n squared) into O(n). The signal is the one-pass complement lookup. Here is the pattern.

Minimum Window Substring is the hard sliding-window problem that tests expand-and-contract with a character-count map. The signal is the grow-to-valid then shrink-to-minimal pattern, checked in O(1) per step.

Group Anagrams tests the 'compute a canonical key and bucket by it' pattern. The signal is choosing a key that anagrams share without comparing every pair. Here is the answer.