OpenAI Software Engineer Interview Guide

Build a key-value store as a single class. Layer on prefix-scan, TTL, then gzip persistence - each phase is its own surface area on top of the previous one.

Serialize a list of arbitrary strings into a single string and parse it back. Any character is allowed in the strings, including the delimiters - the format must round-trip them all.

An in-memory key-value store that checkpoints its full state to a single string and restores from it. Both keys and values are arbitrary strings - delimiters, newlines, unicode, anything. The on-the-wire format must round-trip them all.

A key-value store with timestamped writes. Read at time t returns the latest value written at or before t; later phases add arbitrary-order inserts and range queries.

Tokens issued with a fixed TTL; renew before expiry to extend; count live tokens at time t. Later phases make the count efficient under many issued tokens and add thread safety.

Given a list of half-open meeting intervals, find the minimum number of conference rooms; later phases assign specific room IDs and add capped capacity with priority eviction.

Auto-simulate a turn-based team fight enforcing round order, dying-strike counterattacks, draw detection, and a full time-ordered event log.

A cell becomes infected on step t+1 only if it has at least K infected neighbors right now. Simulate the outbreak, detect when it stalls, and survive boundary and edge shapes.

Convert dotted-quad to 32-bit integers and back, iterate forward and reverse over a range, parse CIDR notation, and decompose an arbitrary range into the minimum CIDR cover.

Count nodes in an n-ary tree using async fan-out: child counts must run concurrently so wall-clock time scales with tree depth, not node count.

An iterator whose state can be captured and replayed - stop iterating, save the state, hand it to a fresh iterator, and continue from exactly where you left off. Layer composition over flat and nested data.

Cells hold literals or formulas that reference other cells. set_cell must evaluate each cell so get_cell is fast; later levels propagate changes to dependents and reject updates that would form a cycle.

A stateful shell that supports POSIX cd semantics - absolute and relative paths, `.`/`..`, symlink resolution, and cycle detection.

Historical-balance lookup (add/deduct stamped at a time, query balance as of t) and FIFO consumption with per-batch expiration - two classes covering the two sides of a credit system.

Single-account GPU credit grants with time windows; FIFO partial spending and windowed balance queries.

A mutable social graph that has to answer point-in-time history without storing a full copy per snapshot, and rank friends-of-friends recommendations both live and as-of a past snapshot.

A single-table in-memory database supporting INSERT and SELECT with WHERE (AND-only equality) and multi-column ORDER BY, with the query results guaranteed deterministic.

Session-based auth: register, login, logout, session TTL, and salted password hashing. Often paired with the TTL token variant.

The chunked-persistence follow-up to KV Store Serialize/Deserialize. Serialize a key-value store with a fixed-width binary (TLV) format, then split the blob into 1 KB chunks and checkpoint it to an S3-style object store - with the chunk count carried in a chunk header, no separate metadata object.

The concurrency follow-up to the in-memory key-value store. Many threads call put / get / erase on one shared store at once; make it correct under contention, then refine from a single global lock to a reader-writer lock and finally to atomic compound operations (get_and_set, compare_and_swap).

Build a web crawler on top of an injected get_links helper - breadth-first crawl with deduplication, then same-domain and fragment handling, then a concurrent version with a thread pool and a shared visited set, and finally depth and global rate limits.

The durability follow-up to the in-memory key-value store. Snapshot the store to disk and restore it, then make the write crash-safe with an atomic temp-file rename, then add an append-only write-ahead log so writes made since the last snapshot survive a crash. Arbitrary key/value bytes must round-trip across disk intact.