hashmap

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 checks whether you reach past sorting for the right structure. The signal is the heap solution (O(n log k)), the bucket-sort O(n) trick, and how it shifts for an unbounded stream. Here is the answer.

Sliding window is one of the highest-yield coding patterns, and this is its canonical problem. The real signal is spotting when a window collapses an O(n squared) scan into one linear pass, and holding the invariant cleanly.

Testing every subarray costs O(n squared). Prefix sums with a remainder-frequency map solve it in a single O(n) pass: two prefixes sharing a remainder mod k enclose a divisible subarray. The pitfall is negative remainders. Here is the clean answer.