data structures

A favorite practical screen at the labs, because it tests concurrency, time handling, and judgment in 20 lines. The trap is the background thread that wastes CPU. Here is the lazy-refill version interviewers want, plus the follow-ups.

The most-asked design-coding question, and a frequent warm-up at the labs before the ML follow-ups. The signal is the hashmap-plus-doubly-linked-list for true O(1), then handling the TTL and concurrency follow-ups cleanly. Here is that build.

A classic that rewards the two-heap insight. A sorted list gives O(n) inserts; two balanced heaps give O(log n) insert and O(1) median. Here is the implementation and the rebalancing detail people get wrong.

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