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Early-stage Startup·Software Engineer·Technical Phone Screen·Junior

JuniorPrefer not to say
Jun 2026

Summary

New grad interview where I got completely destroyed by a theoretical CS question I didn't recognize until after the fact. The interviewer walked me into a trap I had no idea existed and I just... sat there.

Questions Asked (1)

Q1

Write a function that takes another function and its arguments as input, and determines whether that input will cause the function to run forever (infinite loop) or eventually terminate.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

I went through like three different wrong answers before landing on something that was provably broken.

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AI HintsAI Generated

Suggested Approach

First, clarify that this is the halting problem, which is undecidable in general, so no perfect solution exists. Then, discuss practical approaches like static analysis for specific cases (e.g., detecting obvious infinite loops) or dynamic analysis with timeouts, while acknowledging their limitations. Emphasize the theoretical impossibility and the trade-offs between soundness and completeness.

Pro tip: Mention that in practice, you'd use a timeout or resource limits rather than trying to solve the halting problem, and highlight that this is a common interview question to test understanding of computability theory.

1. Clarify the problem

Restate the question to ensure you understand: the function should determine if another function with given arguments will run forever or terminate. Ask if there are any constraints (e.g., time limits, specific language).

2. Identify theoretical limits

Explain that this is the halting problem, proven undecidable by Alan Turing. No general algorithm can solve it for all possible functions and inputs.

3. Discuss practical approaches

Describe possible heuristics: static analysis (e.g., detecting loops without exit conditions), dynamic analysis (running with a timeout), or using theorem provers for restricted cases. Mention that these are incomplete or unsound.

4. Propose a pragmatic solution

Suggest implementing a timeout mechanism: run the function in a separate thread or process with a time limit; if it doesn't finish, assume it might run forever. Acknowledge this doesn't guarantee correctness.

5. Conclude with trade-offs

Summarize that while a perfect solution is impossible, practical solutions involve trade-offs between accuracy, performance, and complexity. Choose based on the use case.

Key Points to Mention

  • Halting problem is undecidable (Turing, 1936)
  • No general algorithm can exist for all functions and inputs
  • Static analysis can detect some infinite loops but is conservative
  • Dynamic analysis with timeouts is practical but not foolproof
  • Trade-offs between soundness (no false positives) and completeness (no false negatives)
  • Real-world applications often use resource limits or sandboxing

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