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Openai·Machine Learning Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say
May 2026

Summary

Went through a coding round at OpenAI for an ML Engineer role and got hit with a simulation problem that looked straightforward but had a lot of hidden complexity once the follow-ups started piling on.

Questions Asked (4)

Q1

Simulate a hero fighting waves of monsters where both sides deal damage each round. For each wave, determine if the hero survives, how much HP is left, and how many rounds it took.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

The core loop wasn't the hard part.

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AI HintsAI Generated

Suggested Approach

Clarify the problem constraints and assumptions, then derive a mathematical formula for rounds and remaining HP per wave. Validate with a simple example and discuss edge cases and trade-offs.

Pro tip: Mention that the number of rounds is ceil(heroHP / monsterDamage) and remaining HP is heroHP - (rounds-1)*monsterDamage, but also consider if the hero attacks first or the monster attacks first, as this affects the outcome.

1. Clarify the problem

Ask about attack order, damage calculation, and whether HP can go negative. Confirm if waves are independent or if hero HP carries over.

2. Derive the math

For a single wave, compute rounds = ceil(heroHP / monsterDamage) if hero attacks first, or ceil((heroHP + monsterDamage - 1) / monsterDamage) if monster attacks first. Remaining HP = heroHP - (rounds-1)*monsterDamage (if hero attacks first) or heroHP - rounds*monsterDamage (if monster attacks first).

3. Validate with an example

Walk through a concrete example (e.g., hero HP=10, monster damage=3) to verify the formula and ensure the hero survives if remaining HP > 0.

4. Handle edge cases

Discuss cases where hero HP <= 0 initially, monster damage = 0, or hero HP exactly divisible by monster damage. Also consider if hero damage affects rounds (if monster has HP).

5. Discuss trade-offs and extensions

Talk about time complexity for multiple waves, potential for simulation vs. formula, and how to handle large numbers or multiple monsters.

Key Points to Mention

  • Attack order (hero first vs. monster first) significantly affects rounds and remaining HP.
  • Use integer arithmetic and ceiling division to avoid floating-point errors.
  • If hero HP is not a multiple of monster damage, the hero survives with positive HP; if it is a multiple, hero HP becomes 0 (or negative if monster attacks first).
  • For multiple waves, hero HP may carry over, so process sequentially.
  • Consider if the hero's damage affects the number of rounds (if monster has HP).
  • Edge cases: zero damage, zero HP, and large numbers requiring efficient computation.

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

Q2

Given multiple possible orderings of monster waves, how would you find the ordering that lets the hero survive the most waves or finish in the fewest rounds?

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

This is where things got interesting.

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AI HintsAI Generated

Suggested Approach

First, clarify the problem constraints and define the objective precisely (maximize waves survived or minimize rounds). Then, model it as an optimization problem over permutations, likely reducible to a scheduling problem, and propose an algorithm (e.g., greedy with exchange argument or DP) with complexity analysis.

Pro tip: Discuss the trade-off between optimality and efficiency: for large N, a greedy heuristic may be necessary, but prove its optimality under certain conditions or provide approximation guarantees.

1. Clarify problem and constraints

Ask about wave properties (e.g., fixed order within a wave, hero's state changes), objective (max waves vs min rounds), and input size. This ensures you solve the right problem.

2. Formalize as an optimization problem

Define variables, state transitions, and objective function. Recognize it as a permutation optimization, possibly equivalent to scheduling with precedence or resource constraints.

3. Identify problem structure

Look for properties like monotonicity, exchange arguments, or optimal substructure. For example, if waves have independent effects, a greedy sort by some key might work.

4. Propose algorithm and analyze

Describe a concrete algorithm (e.g., DP over subsets, greedy with proof, or heuristic) and analyze time/space complexity. Discuss optimality or approximation ratio.

5. Discuss trade-offs and extensions

Compare exact vs heuristic approaches, scalability, and potential ML integration (e.g., learning a policy). Mention edge cases and testing strategy.

Key Points to Mention

  • Problem reduction to known problems (e.g., scheduling, TSP, knapsack)
  • Greedy algorithms with exchange argument for optimal ordering
  • Dynamic programming over subsets for small N
  • Complexity analysis (time and space) and scalability
  • Trade-offs between optimality and computational efficiency
  • Potential use of machine learning (e.g., reinforcement learning) for large-scale or uncertain scenarios

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

Q3

How would you extend the simulation to handle special abilities like healing, shields, or area-of-effect attacks, and how do you decide optimal targeting each round?

Algorithms & Data StructuresSystem Design
Author's notes

Honestly the most fun part of the problem.

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AI HintsAI Generated

Suggested Approach

Start by outlining a modular architecture that separates core simulation logic from ability effects, then discuss how to model abilities as composable actions with targeting constraints. For optimal targeting, frame it as a decision problem under uncertainty, comparing search-based methods (e.g., minimax, MCTS) with learned policies (e.g., reinforcement learning) and explaining trade-offs.

Pro tip: Emphasize that optimal targeting often depends on the objective (e.g., maximize damage vs. minimize risk) and that a hybrid approach—using search for short horizons and learned value functions for long-term planning—can balance optimality and scalability.

1. Define ability mechanics and effects

Specify how each ability modifies state: healing restores HP, shields absorb damage, AoE affects multiple targets within a radius. Represent abilities as data-driven actions with parameters (cost, range, cooldown).

2. Design extensible simulation architecture

Use an entity-component-system (ECS) or similar pattern to decouple abilities from entities. Implement an effect system that applies changes to game state, allowing new abilities to be added without modifying core logic.

3. Model targeting as an optimization problem

Formulate targeting as maximizing a utility function (e.g., expected damage, survival probability) over possible targets. Consider constraints like range, line-of-sight, and resource costs.

4. Choose and justify targeting algorithms

For small state spaces, use exhaustive search or minimax; for large, use Monte Carlo Tree Search (MCTS) or reinforcement learning. Discuss how to handle partial observability and stochastic outcomes.

5. Evaluate and iterate

Define metrics (e.g., win rate, average reward) and test against baselines. Use simulation to generate data for training ML models, and consider online learning to adapt to opponent behavior.

Key Points to Mention

  • Composability and modularity: abilities as independent modules that can be combined.
  • State representation: how to encode HP, shields, positions, and cooldowns for efficient computation.
  • Utility functions: designing reward signals that capture short-term and long-term goals.
  • Search vs. learning: trade-offs between optimality, computational cost, and generalization.
  • Handling stochasticity: using expectimax or probabilistic models for uncertain outcomes.
  • Scalability: techniques like pruning, heuristics, or function approximation for large action spaces.

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

Q4

What are the minimum hero HP and attack stats required to clear all waves without dying?

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

Binary search on HP and ATK separately, running the simulation as the check function.

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AI HintsAI Generated

Suggested Approach

Clarify the problem as a dynamic programming or binary search optimization: for a given HP and attack, simulate the game to check if all waves can be cleared without dying. Then binary search on HP and attack to find the minimum pair that works, or use DP to compute the Pareto frontier of feasible (HP, attack) pairs.

Pro tip: Discuss the trade-off between HP and attack: increasing attack reduces the number of enemy turns, which indirectly reduces required HP. This interdependence means you can't optimize them independently; you need to search over both dimensions or use a multi-objective optimization approach.

1. Clarify the problem and assumptions

Ask about game mechanics: turn order, damage calculation, wave composition, and whether HP/attack can be upgraded between waves. Confirm that 'without dying' means HP > 0 at all times.

2. Define a feasibility check

Given fixed HP and attack, simulate the game wave by wave, ensuring the hero survives each wave. This check runs in O(total enemies) time.

3. Choose an optimization strategy

If the search space is small, brute-force all pairs. Otherwise, use binary search on one stat while computing the minimum required other stat via simulation, or use dynamic programming to find the Pareto-optimal frontier.

4. Analyze complexity and trade-offs

Compare approaches: binary search with simulation (O(N log M) where N is enemies, M is stat range) vs. DP (O(N * HP * ATK) if discretized). Discuss memory vs. time trade-offs.

5. Validate with edge cases

Test with minimal stats, single enemy, multiple waves, and scenarios where attack is very high or very low. Ensure the solution handles ties and returns the minimum pair correctly.

Key Points to Mention

  • Dynamic programming for state-space search or Pareto frontier computation
  • Binary search on one variable with a simulation-based feasibility check
  • Time and space complexity analysis of different approaches
  • Trade-off between HP and attack: higher attack reduces required HP
  • Game simulation mechanics: turn order, damage per turn, wave progression
  • Edge cases: zero attack, infinite HP, multiple enemies with varying stats

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