Arrays, Hashing, and Iterators

Coinbase leans on applied, build-a-system rounds, but a lighter algorithmic tier still rides alongside them. The opening question of the screen is usually a warmup, and the pair-programming round often starts with a small reusable component before it escalates. The questions that show up here cluster around two tools: a hash map for constant-time lookup, and an iterator that pulls from several sources one element at a time. Neither is about a clever trick. Both are about a clean abstraction that an interviewer can read and extend.

Both patterns trade a scan for a lookup. A hash map turns "scan everything to find the match" into "ask the map once". An iterator turns "merge the lists up front" into "hand back the next element when asked, and keep just enough state to find the one after it". A design-a-structure problem combines the two: a map for the key, an ordering for the history. The three patterns below carry almost every warmup and component round Coinbase asks, and each runnable problem is one engineers see in a Coinbase loop.