Group anagrams

Anagrams share either the same sorted characters or the same 26-character frequency vector. For each word, build one of those signatures and append the word to a map bucket keyed by it. Sorting each word gives O(n * k log k) for n words of length k; a frequency signature gives O(n * k) for lowercase English letters. The output order is usually unconstrained, but do not use a mutable frequency array itself as an unsafe map key. In an interview, make the invariant explicit before writing code, trace a boundary example that breaks a tempting shortcut, state what each stored value represents, and explain why every update preserves correctness. This verification step also surfaces empty-input, duplicate-value, and numeric-limit cases.