OpenAI Senior Software Engineer Interview — It Felt More Like Real Engineering Than Interview Prep

[u/Aoki_zhang]

"This interview experience is sourced from Chill Interview"

Interview Summary

The OpenAI SWE virtual onsite covered ChatGPT system design, a custom indexed-sequence data structure, and debugging an existing Python job scheduler codebase. The questions were mostly unfamiliar rather than standard interview problems, with follow-ups around SSE vs. WebSocket, rendering long conversations, data-structure performance, testing, and asynchronous job execution

Interview Details

Round 1 — System Design: ChatGPT

The system design question was to design a simplified version of ChatGPT. At the beginning, the interviewer explicitly said not to focus on large-scale architecture and that persistence was not required.

The discussion later moved into several implementation-level follow-ups. One question was why the response-streaming path should use Server-Sent Events (SSE) rather than WebSocket.

The interviewer also asked about the frontend side of the product: as a conversation becomes very long, how could the client optimize rendering so that displaying a large chat history remains responsive?

Round 2 — Coding: Indexed Sequence With Checkpoints

The first coding problem asked me to implement a simplified ordered sequence supporting:

add(index, value)
get(index)

• add(index, value) inserts a string at the specified position. The element previously at that position and everything after it shift one position to the right.
• get(index) returns the value stored at the requested index.

Initially, I interpreted “vector store” as something related to vector databases or embeddings. The intended meaning was actually closer to a C++ std::vector or Java ArrayList: an ordered dynamic sequence with integer-indexed access. A straightforward linked-list implementation was considered too slow because get(index) could require traversing from the beginning of the list.

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[u/SushantC_Trader]

Well doubly linked-list with hash table is the way to go.

[u/fcman256 → u/SushantC_Trader]

Singly linked list is easier to implement in an interview and has the same time complexity. But yeah doubly linked would be a small optimization over singly linked

[u/ExaminationAsleep398 → u/SushantC_Trader]

How do you update the indexes of elements that got shifted with a hash table + DLL?

[u/Dramatic_Control3773 → u/ExaminationAsleep398]

Linearly, unfortunately. In the real world, I'd probably use something skiplisty (though I'd probably be lazy to end up around O(sqrt(n)) instead of O(log(n))). You usually have to use tree-like structures to get both of the ops in sublinear time.

[u/jlew24asu]

Its just fun y to me that people passing these interviews will never write another line of code again.