The Implementation Round

The xAI coding round often hands you a single component to build cleanly, correctly, and fast enough to defend under questioning. The companion lesson works the components that grow a live extension: the transactional key-value store, the checkpointable iterator, and the multi-level rate limiter. This lesson works the other four the round keeps returning to, usually graded on a clean first build and its complexity with no bolted-on twist. They are a cache with O(1) reads and writes and a task manager that tracks work broken into sub-tasks. The last two are a versioned array you can read at any past snapshot and a Premium question over a grid backed by a trie.

The xAI angle

Reach for the structure that makes the graded operation cheap, and name it before you type. Each of these four has an obvious slow version and one data structure that turns the hard operation into a few lines. An LRU cache is O(1) only with a hash map pointing into a doubly linked list, and a Premium question is a tree, so completing a branch is a walk over children. A Premium question is cheap only if each index keeps its own short version log, so the whole array never gets copied on a snap. A Premium question over many dictionary words is fast only when the words share a trie, so one grid walk checks them all at once. The interviewer watches which structure you pick, because the pick is the answer and the code just transcribes it.