DoorDash·Software Engineer·Technical Phone Screen·Senior

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May 2026

Summary

DoorDash software engineering round focused on a broken round-robin scheduler implementation. You had to read the code, find the bugs, explain them, and fix them. More of a debugging and reasoning exercise than a from-scratch coding problem, which I didn't expect.

Questions Asked (1)

You're given a buggy round-robin scheduler implementation. Identify the defects (such as off-by-one errors in pointer advancement, broken empty-queue handling, unfair tie-breaking, race conditions, or wrong time-quantum accounting), explain why each one is wrong, fix them, and walk through complexity and edge cases.

Algorithms & Data StructuresTechnical Trade-offsSystem Design

Author's notes

This one took me a minute to settle into.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the scheduler's requirements (e.g., time quantum, queue behavior) and then systematically review the code for common defects like off-by-one errors, empty-queue handling, and race conditions. For each defect, explain the impact, propose a fix, and analyze the complexity and edge cases to ensure robustness.

Pro tip: Demonstrate a methodical debugging approach by writing test cases for edge scenarios (e.g., empty queue, single process, simultaneous arrivals) before diving into fixes. This shows foresight and reduces the risk of overlooking subtle bugs.

1. Clarify Requirements and Assumptions

Ask clarifying questions about the scheduler's expected behavior, such as time quantum, queue management, and concurrency model. Confirm assumptions about input and output.

2. Identify Defects Systematically

Review the code for common pitfalls: off-by-one in pointer advancement, empty-queue handling, unfair tie-breaking, race conditions, and incorrect time-quantum accounting. List each defect with a brief explanation.

3. Explain Why Each Defect is Wrong

For each defect, describe the incorrect behavior it causes and its impact on fairness, correctness, or performance. Relate it to the scheduler's requirements.

4. Propose and Implement Fixes

Suggest specific code changes to fix each defect, ensuring they align with the clarified requirements. Consider trade-offs between different solutions.

5. Analyze Complexity and Edge Cases

Discuss the time and space complexity of the fixed scheduler and walk through edge cases like empty queue, single process, multiple processes with same priority, and concurrent access.

Key Points to Mention

Off-by-one errors in pointer advancement can cause processes to be skipped or repeated, leading to unfairness.
Empty-queue handling must avoid null pointer dereferences and ensure the scheduler doesn't crash or spin.
Unfair tie-breaking can starve processes; use a deterministic policy like FIFO or round-robin order.
Race conditions in concurrent environments require synchronization mechanisms like locks or atomic operations.
Time-quantum accounting must correctly decrement remaining time and handle preemption accurately.
Complexity analysis: O(1) for enqueue/dequeue with a circular queue, O(n) for scanning if using a list; edge cases include zero quantum, negative time, and process arrival during execution.

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