Openai·Software Engineer·Technical Phone Screen·Senior

Senior

Apr 2026

Summary

Interviewed for a software engineering role at OpenAI and got hit with a stateful balance-tracking problem that looked deceptively manageable at first glance. The sticky-None behavior is where things get interesting and where I almost tripped up.

Questions Asked (1)

You have a table of adjustments with an id, a numeric value, a start time, and an optional stop time. Write a function check(t) that returns the sum of all active entries at time t, where an entry is active if start <= t < stop (no stop means it's still active). Return None if the total is negative, return 0 if nothing is active, and once any call has returned None, all future calls must also return None regardless of what the actual total would be.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The interval logic and the None-for-negative part were fine, I got those pretty quickly.

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AI HintsAI Generated

Suggested Approach

First, clarify the requirements and edge cases, especially the sticky None behavior. Then, design a data structure that efficiently computes the sum of active intervals at any time t, such as sorting intervals and using a sweep line or prefix sums. Finally, implement the function with careful handling of the sticky None flag and discuss trade-offs.

Pro tip: Mention that the sticky None can be implemented as a simple boolean flag that short-circuits all future calls, and emphasize that this is a stateful requirement that must be handled carefully in concurrent scenarios.

1. Clarify requirements and edge cases

Ask about input types, interval inclusivity, and the exact semantics of the sticky None. Confirm whether the function is called multiple times and if the table can be preprocessed.

2. Choose an efficient data structure

Consider sorting intervals by start time and using a sweep line or prefix sums to answer queries in O(log n) or O(1) after O(n log n) preprocessing. Discuss trade-offs between preprocessing time and query time.

3. Implement the check function

Write code that computes the sum of active intervals at time t, handles the sticky None flag, and returns None, 0, or the sum as appropriate. Ensure the flag is set when the sum is negative.

4. Test with edge cases

Test with no active intervals, negative sums, intervals with no stop, and multiple calls to verify sticky None. Also test boundary conditions like t exactly at start or stop.

5. Discuss scalability and concurrency

If relevant, mention how the solution scales with large datasets and how the sticky None flag would behave in a multi-threaded environment, suggesting synchronization if needed.

Key Points to Mention

Interval activity condition: start <= t < stop, with no stop meaning active indefinitely.
Sticky None: once a call returns None, all subsequent calls must return None, requiring a persistent flag.
Efficient querying: preprocess intervals (e.g., sort and use sweep line or prefix sums) to avoid O(n) per query.
Edge cases: empty table, no active intervals, negative sum, and boundary times.
Trade-offs: preprocessing time vs. query time, memory usage, and simplicity vs. performance.
Concurrency: if multiple threads call check, the sticky None flag needs synchronization.

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