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Openai·Software Engineer·Onsite - System Design / Architecture·Staff

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Jun 2026

Summary

System design round at OpenAI, one big question that ate the whole session. The kind of problem where you think you know where it's going and then it keeps expanding.

Questions Asked (1)

Q1

Design a distributed system to fill a 50x50 crossword puzzle with 100 word slots, drawing from a dictionary of 1 million words. Show why a single machine can't do this at scale, then walk through a multi-machine solution covering task splitting, distributed backtracking, work stealing, pruning, communication overhead, and termination detection. Also compare with stochastic approaches like simulated annealing.

System DesignAlgorithms & Data StructuresTechnical Trade-offs
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AI HintsAI Generated

Suggested Approach

Start by quantifying the computational complexity and memory requirements to justify why a single machine is infeasible. Then outline a distributed architecture that partitions the search space, uses backtracking with pruning, and incorporates work stealing for load balancing. Finally, discuss termination detection and compare with stochastic methods like simulated annealing, highlighting trade-offs.

Pro tip: Emphasize that the goal is not just parallelism but efficient search space exploration; mention that pruning and heuristics are as important as distribution. Also, note that stochastic methods can find good solutions faster but may not guarantee optimality, so a hybrid approach might be best.

1. Quantify the problem and single-machine limits

Estimate the search space size (e.g., 100 slots, 1M words, constraints) and memory/CPU requirements. Show that backtracking on a single machine would take years and exceed memory.

2. Design distributed backtracking with task partitioning

Split the crossword into subproblems (e.g., by assigning words to slots or partitioning the grid). Use a master-worker or peer-to-peer model where workers perform backtracking on assigned subproblems.

3. Incorporate pruning and heuristics

Apply constraint propagation, forward checking, and word frequency heuristics to reduce the search space. Prune branches early based on letter constraints and dictionary filtering.

4. Implement work stealing and communication

Use a work-stealing scheduler (e.g., random stealing or queue-based) to balance load. Minimize communication overhead by batching updates and using efficient protocols (e.g., gossip).

5. Termination detection and comparison with stochastic methods

Use a distributed termination algorithm (e.g., Dijkstra's or credit-based) to detect when all workers are idle and no solution exists. Compare with simulated annealing: stochastic methods are simpler but may not guarantee a solution; backtracking is exact but harder to distribute.

Key Points to Mention

  • Computational complexity: 100 slots with 1M words leads to an astronomical search space (e.g., 1M^100 combinations), infeasible for a single machine.
  • Memory constraints: storing the dictionary and search state may exceed single-machine RAM, especially with backtracking stacks.
  • Task splitting strategies: partition by grid regions, word assignments, or use a work queue of partial solutions.
  • Work stealing: dynamic load balancing to handle uneven subproblem difficulty, with minimal communication overhead.
  • Pruning techniques: constraint satisfaction (e.g., arc consistency), dictionary indexing by length and pattern, and early conflict detection.
  • Termination detection: distributed algorithms like Dijkstra-Scholten or credit-based to know when to stop, especially if no solution exists.
  • Stochastic approaches: simulated annealing can explore large spaces but may get stuck in local optima; hybrid approaches combine both.

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