hash map

The textbook warm-up that tests for the hash-map instinct: swap space for time to turn O(n squared) into O(n). Interviewers look for the one-pass complement lookup. The pattern follows.

Minimum Window Substring is the hard sliding-window problem that probes expand-and-contract with a character-count map. Interviewers look for the grow-to-valid then shrink-to-minimal pattern, verified in O(1) per step.

Group Anagrams probes the 'compute a canonical key and bucket by it' pattern. Interviewers look for choosing a key that anagrams share without comparing every pair. The answer follows.