Openai·Machine Learning Engineer·Technical Phone Screen·Senior

Senior

Jun 2026

Summary

Got a coding question for an ML Engineer role at OpenAI that was basically a grid simulation problem with a twist. Pretty algorithmic, not much ML flavor to it, but that seems to be their style.

Questions Asked (1)

Extend a disease-spread grid simulation to include an immune cell type that can neither be infected nor transmit infection. Return the number of days until the grid reaches a stable state with no new infections.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The immune cell addition sounds minor but it actually changes how you think about termination conditions.

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AI HintsAI Generated

Suggested Approach

Model the grid as a graph and simulate daily spread using BFS, treating immune cells as barriers that block infection. Track newly infected cells each day and stop when no new infections occur, returning the day count.

Pro tip: Clarify the infection rules upfront (e.g., 4-directional spread, infection after one day) and discuss how immune cells affect connectivity, showing you consider edge cases and scalability.

1. Clarify rules and assumptions

Confirm grid dimensions, neighbor directions, infection timing, and immune cell behavior (e.g., cannot be infected or transmit).

2. Choose data structures

Use a 2D array for the grid and a queue for BFS to process cells day by day, marking states (susceptible, infected, immune).

3. Simulate day-by-day spread

For each day, process all currently infected cells, attempt to infect susceptible neighbors, and collect newly infected cells for the next day.

4. Detect stability and count days

Stop when a day produces no new infections; return the number of days elapsed since the start.

5. Analyze complexity and trade-offs

Discuss time/space complexity (O(N*M) per day, O(N*M) total) and potential optimizations like multi-source BFS or early termination.

Key Points to Mention

BFS or level-order traversal to simulate daily spread
Immune cells as obstacles that block infection paths
Tracking newly infected cells to avoid redundant checks
Stability condition: no new infections in a day
Time and space complexity analysis
Edge cases: no initial infected, all immune, disconnected regions

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