trie

Autocomplete is the canonical trie question, and it checks whether you reach for the right structure rather than scanning a word list. The signal is O(prefix) lookup, the DFS to gather completions, and the follow-ups (ranking, memory). Here is the build.

Every keystroke has to return ranked suggestions in tens of milliseconds, at search-engine QPS. The interesting part is the data structure and what you precompute, not the query path. Here is the design that holds up under the latency budget.