design

The gentlest 'design a class' question in FDE screens, and the warm-up interviewers use before rate limiters and sessionization. The deque trick is easy; the API and time-window follow-ups are where the signal is.

The single most-asked design-a-data-structure question in FDE loops. There's an interview-legal Python shortcut and a from-scratch version, and knowing when to offer which is half the grade.

A tree question that's secretly a format-design question, which is why FDE loops love it. The preorder-with-null-markers trick is clean, but the delimiter and malformed-input follow-ups are where offers are decided.

A multi-part favorite at OpenAI-style screens: simple store, then time travel, then deletes that don't actually delete. The append-only insight plus one bisect call solves the whole thing, if you set up the invariant correctly.

A verbatim-reported multi-part at OpenAI and Anthropic. Each level breaks the previous design on purpose: the burst-at-the-boundary flaw is planted, and naming it before the interviewer does is how you win the round.

A three-level build that quietly tests whether your L1 design survives L3. Regex substitution wins level one and loses the round; here's the token-based structure that carries through conditionals and loops.

Exact-match pub/sub is a dict of lists. The round is decided at level two (wildcard matching on hierarchical topics) and at level three, when the interviewer asks what happens when a subscriber's callback throws.

The OpenAI FDE signature question, reported near-verbatim for two years: four levels in ~60 minutes, where L4 silently breaks every naive L3 implementation. The lazy-expiry design that survives all four levels, with the pacing plan.

The other OpenAI signature build, reported in four parts for two years. Part C silently punishes anyone who hardcoded single-condition filtering in part B; here's the predicate-compiler design that absorbs all four parts.

The OpenAI variant where the data structure is the warm-up: the round is graded on whether your tests control time and whether you can argue global vs per-key vs optimistic locking with actual reasons. Most candidates ace part one and lose the round in parts two and three.

A reported OpenAI build that looks like billing and is actually a data-structure question in disguise: credit grants that expire, usage that must burn the right grant first, and balance queries at arbitrary times. The earliest-expiry-first invariant carries the whole problem.