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Openai·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say
Jun 2026

Summary

Got a coding round at OpenAI for a SWE role and the problem was way more involved than I expected. It was a turn-based monster battle simulator where you had to find the optimal strategy across ordering and revival decisions, basically a full search problem dressed up as a game.

Questions Asked (3)

Q1

Design and implement an optimal turn-based monster battle simulator where you control deployment order and revival decisions to maximize win probability across all possible battle outcomes.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs
Author's notes

This one took me a minute to even parse.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem scope and constraints, then propose a modeling approach that captures the battle mechanics and decision points. Discuss algorithmic strategies (e.g., dynamic programming, game theory, simulation) and system design considerations for scalability and optimality.

Pro tip: Emphasize that 'optimal' depends on the objective (e.g., maximizing win probability) and constraints (e.g., time, memory), and propose a hybrid approach combining exact methods for small cases and heuristics for large ones.

1. Clarify Requirements and Constraints

Ask questions to understand the battle mechanics, monster attributes, turn order, revival rules, and what 'optimal' means (e.g., maximize win probability). Identify input size limits and performance requirements.

2. Model the Problem

Formalize the battle as a state space with actions (deployment, revival) and stochastic outcomes. Define states, transitions, and reward (win/loss).

3. Choose Algorithmic Approach

Propose using dynamic programming or game tree search with memoization for exact solutions on small inputs. For larger inputs, suggest Monte Carlo Tree Search (MCTS) or heuristic-based simulation to approximate optimal play.

4. Design System Architecture

Outline a modular system: battle engine, decision module, and simulation runner. Discuss trade-offs between precomputation and real-time decision-making, and how to handle concurrency and scalability.

5. Evaluate and Iterate

Describe how to test the solution: unit tests for battle mechanics, performance benchmarks, and comparison against baseline strategies. Discuss potential optimizations and extensions.

Key Points to Mention

  • State space representation and transition modeling
  • Dynamic programming / memoization for optimal decisions
  • Game theory concepts (e.g., minimax, expectimax) for adversarial or stochastic outcomes
  • Monte Carlo simulation for approximating win probabilities
  • Trade-offs between optimality, time complexity, and memory usage
  • System design considerations: modularity, scalability, and real-time constraints

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

Q2

How would you represent the battle state efficiently to support memoization and pruning when searching for the optimal strategy?

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

They pushed pretty hard on this after I had a working brute-force sketch.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the game's state space and the search algorithm (e.g., minimax with alpha-beta pruning). Then propose a compact, canonical representation that captures only the essential information for memoization, such as bitboards or immutable tuples, and explain how it enables efficient hashing and pruning.

Pro tip: Emphasize that the representation must be canonical (e.g., normalizing symmetric states) to maximize memoization hits, and discuss the trade-off between memory and speed when choosing between bit-packing and object-based states.

1. Identify essential state variables

List all information needed to determine the game's outcome and legal moves, such as piece positions, turn, and any game-specific counters.

2. Choose a compact encoding

Select a representation like bitboards, tuples, or Zobrist hashing that minimizes memory and enables fast equality checks and hashing.

3. Ensure canonicalization

Normalize equivalent states (e.g., via symmetry reduction) to increase memoization effectiveness and reduce redundant search.

4. Integrate with search algorithm

Explain how the representation supports memoization (e.g., transposition tables) and pruning (e.g., alpha-beta bounds) without excessive overhead.

5. Discuss trade-offs

Compare alternatives (e.g., bit-packing vs. object graphs) in terms of memory, speed, and implementation complexity, and justify your choice.

Key Points to Mention

  • Bitboards for board games: compact, fast bitwise operations, and easy hashing.
  • Zobrist hashing for incremental updates and transposition tables.
  • Canonicalization: symmetry reduction (e.g., rotations/reflections) to merge equivalent states.
  • Immutable data structures to avoid accidental mutation and enable safe caching.
  • Trade-offs: memory vs. speed, and the impact of state size on pruning efficiency.
  • Integration with alpha-beta pruning: storing bounds and best moves in transposition tables.

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

Q3

How would you extend your simulator to support a revival mechanism where defeated monsters can be brought back under certain conditions, and how does that change your search strategy?

System DesignAdaptability & AmbiguityTechnical Trade-offs
Author's notes

The part that tripped me up was that revival introduces a decision node into what was otherwise a deterministic path.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the revival conditions and simulator architecture, then propose a design that integrates revival as a state transition, and finally analyze how it affects the search strategy, likely requiring a more complex state representation and search algorithm. Emphasize trade-offs between accuracy and performance.

Pro tip: Discuss how revival introduces cycles in the state space, which can break naive search algorithms; mentioning techniques like iterative deepening or memoization with state hashing shows depth.

1. Clarify Requirements

Ask questions to understand revival conditions (e.g., time-based, item-based) and constraints (e.g., limited revivals). This ensures the design meets the intended use case.

2. Extend Simulator State

Incorporate revival-related state variables (e.g., revival counters, cooldowns) into the simulator's state representation, and define transition rules for revival.

3. Adapt Search Strategy

Modify the search algorithm to handle the expanded state space and potential cycles; consider algorithms like A* with a heuristic that accounts for revival, or Monte Carlo Tree Search for stochastic revivals.

4. Analyze Trade-offs

Evaluate performance implications (e.g., increased branching factor, memory usage) and discuss mitigation strategies like pruning or approximation.

5. Validate and Iterate

Propose testing the extended simulator with unit tests and benchmarks, and iterating on the design based on results.

Key Points to Mention

  • State space explosion due to revival mechanics and how to manage it (e.g., state abstraction, symmetry reduction).
  • Cycle detection and handling in search algorithms (e.g., using visited sets with state hashing).
  • Choice of search algorithm: BFS/DFS vs. A* vs. MCTS, and why one is more suitable for revival scenarios.
  • Trade-offs between optimality, completeness, and computational resources.
  • Potential need for heuristics that estimate the value of revival opportunities.
  • Impact on simulator performance and scalability, and possible optimizations like caching or parallelization.

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