Openai·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

Jun 2026Remote

Summary

OpenAI software engineer coding round, basically a credit balance system with a twist I didn't see coming until I was already halfway through a wrong implementation.

Questions Asked (1)

Design a credit balance system that supports add, subtract, and getBalance operations for users, where a failed subtract (due to insufficient funds) causes the next getBalance call to return None instead of the actual balance.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

I had the basic balance tracking done in like five minutes and thought I was cruising.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements and edge cases, especially the unusual behavior where a failed subtract poisons the next getBalance. Then design a data structure that tracks each user's balance and a flag indicating whether the last subtract failed. Implement the operations with careful state management, and discuss trade-offs like concurrency and persistence.

Pro tip: Explicitly call out the 'poisoning' behavior as a stateful side effect and propose how to handle it cleanly (e.g., using a per-user flag that is reset after getBalance). This shows you think about state consistency and failure recovery, which is crucial for production systems.

1. Clarify requirements and edge cases

Ask questions to confirm: Is the system per-user? What should happen if subtract is called with insufficient funds multiple times? Should the flag persist across multiple getBalance calls? What about concurrent access?

2. Design the data model

Propose a data structure, e.g., a map from user ID to an object containing balance and a boolean flag (e.g., 'lastSubtractFailed'). Explain how the flag is set on failed subtract and reset on successful subtract or after getBalance.

3. Implement operations with state transitions

Walk through the logic for add, subtract, and getBalance. For subtract: if amount > balance, set flag and return failure; else update balance and clear flag. For getBalance: if flag is set, return None and reset flag; else return balance.

4. Discuss trade-offs and extensions

Address concurrency (e.g., locks or atomic operations), persistence (database schema), and whether the flag should be per-user or global. Mention alternative designs like using exceptions or returning error codes instead of a flag.

5. Test with examples

Provide a few test cases: normal add/subtract, failed subtract followed by getBalance returning None, and subsequent getBalance returning actual balance. Show how the flag resets.

Key Points to Mention

State management: using a per-user flag to track failed subtracts and resetting it appropriately.
Concurrency: ensuring thread-safety with locks or atomic operations, especially for the flag and balance updates.
Persistence: how to store the flag and balance in a database, and whether the flag should survive restarts.
Trade-offs: alternative designs like throwing exceptions, returning error codes, or using a separate error state.
Edge cases: multiple failed subtracts, subtract after getBalance, and behavior when balance is exactly zero.
API design: whether getBalance should have side effects (resetting the flag) and how to document that clearly.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.