Debugging Testing and Optimization Questions

Core engineering skills for identifying, diagnosing, testing, and improving code correctness and performance. Covers approaches to finding and fixing bugs including reproducible test case construction, logging, interactive debugging, step through debugging, and root cause analysis. Includes testing strategies such as unit testing, integration testing, regression testing, test driven development, and designing tests for edge cases, boundary conditions, and negative scenarios. Describes performance optimization techniques including algorithmic improvements, data structure selection, reducing time and space complexity, memoization, avoiding unnecessary work, and parallelism considerations. Also covers measurement and verification methods such as benchmarking, profiling, complexity analysis, and trade off evaluation to ensure optimizations preserve correctness and maintainability.

MediumTechnical

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You are reviewing a teammate's pull request that adds memoization to a function. What checks would you perform in code review to ensure correctness, memory safety, and thread-safety? Provide concrete review comments you might leave.

HardTechnical

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A recent performance patch reduced average latency but increased p99 latency. How would you investigate and resolve this regression while preserving average-case gains? Describe analysis steps and possible code or system fixes.

MediumTechnical

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Write a small C function that leaks memory when called repeatedly, then explain how you would detect and fix the leak using valgrind and heap analysis. Provide code snippet and high-level commands you would run.

MediumTechnical

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Explain how you would profile and optimize a heavily used SQL JOIN that causes CPU spikes in the DB. Discuss rewriting queries, adding indexes, denormalization, materialized views, and when to move computation out of the DB.

HardTechnical

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Implement a deterministic test harness for asynchronous JavaScript code that processes a queue in the background (Node.js). Show how you would structure tests to avoid flakiness, mock timers, and assert processing order for multiple queued tasks.

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