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Openai·Software Engineer·Onsite - Coding / Algorithms·Senior

Senior
May 2026

Summary

Coding exercise at OpenAI for a software engineer role, 60 minutes to design and implement a turn-based two-player game simulation from scratch. The scope was broader than I expected going in.

Questions Asked (1)

Q1

Design and implement a two-player turn-based game simulation (like tic-tac-toe) that models game state, handles move validation, detects win/loss/draw conditions, and can predict the winner from any mid-game state. The design should be extensible to other similar games.

System DesignAlgorithms & Data StructuresTechnical Trade-offs
Author's notes

60 minutes felt fine until I got to the winner prediction part and realized I'd painted myself into a corner with my class structure.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and defining the game's core abstractions (state, moves, rules) before diving into implementation. Propose a modular design with clear interfaces for extensibility, then discuss algorithms for move validation, win detection, and winner prediction (e.g., minimax with memoization). Balance trade-offs between simplicity and generality, and be ready to code key components.

Pro tip: Demonstrate extensibility by designing a generic game framework (e.g., using the Strategy pattern for rules) and mention how to optimize winner prediction with alpha-beta pruning or transposition tables. Also, proactively discuss testing and edge cases to show production-level thinking.

1. Clarify Requirements and Scope

Ask questions to understand constraints: board size, game rules, performance needs, and extensibility goals. Confirm whether the simulation should support AI players or just human moves.

2. Define Core Abstractions and Interfaces

Outline classes/interfaces for GameState, Move, Player, and GameRules. Emphasize separation of concerns and how new games can be added by implementing these interfaces.

3. Design Game State and Move Validation

Describe how to represent the board (e.g., 2D array), track turns, and validate moves (e.g., check occupancy, bounds, and rule-specific constraints). Mention immutable state for easier prediction.

4. Implement Win/Loss/Draw Detection

Explain algorithms to check for winning conditions (e.g., lines, diagonals) and draw (full board). Discuss efficiency and how to generalize for different win patterns.

5. Implement Winner Prediction and Extensibility

Describe using minimax with alpha-beta pruning for perfect play, and memoization for performance. Discuss how the design supports other games (e.g., Connect Four) by swapping rule implementations.

Key Points to Mention

  • Use of design patterns (e.g., Strategy, Factory) for extensibility
  • Immutable game state to simplify prediction and avoid side effects
  • Minimax algorithm with alpha-beta pruning and memoization for winner prediction
  • Clear separation of game rules from game state and move logic
  • Efficient win detection (e.g., checking only affected lines after a move)
  • Testing strategy: unit tests for move validation, win detection, and prediction

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