data structures

Labs reach for this practical screen often, since it probes concurrency, time handling, and judgment inside 20 lines. The pitfall is a background thread that burns CPU. Below is the lazy-refill approach interviewers expect, along with the follow-ups.

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

A classic that pays off the two-heap insight. A sorted list costs O(n) per insert; 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 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.