DoorDash·Software Engineer·Onsite - Coding / Algorithms·Intermediate

IntermediatePrefer not to say

Jun 2026

Summary

DoorDash coding round focused entirely on debugging a pre-written 'Dasher Map' system rather than writing anything from scratch. Three distinct bugs to find and fix, which sounds manageable until you're actually staring at someone else's broken code under time pressure.

Questions Asked (1)

You're given a buggy 'Dasher Map' system with three known issues: broken randomization, a misconfigured hashmap at initialization, and null pointer errors from faulty key-swapping logic. Find each bug, explain why it's happening, and apply minimal fixes. Add tests where appropriate.

Algorithms & Data StructuresRoot Cause AnalysisTechnical Trade-offs

Author's notes

The randomization bug tripped me up first because I kept looking in the wrong place.

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AI HintsAI Generated

Suggested Approach

Start by reproducing each bug with a minimal test case to confirm the symptoms, then trace the root cause in the code, and finally apply the smallest possible fix while adding regression tests. Prioritize the null pointer errors first as they likely cause crashes, then address the hashmap misconfiguration, and finally the randomization issue.

Pro tip: Demonstrate a systematic debugging process: always write a failing test before fixing a bug, and after the fix, ensure the test passes and no other tests break. This shows you value reliability and maintainability.

1. Reproduce and Prioritize

Write minimal test cases to reproduce each bug and determine their impact. Prioritize null pointer errors as they cause crashes, followed by hashmap misconfiguration, then randomization.

2. Root Cause Analysis

For each bug, trace the code to identify the exact cause. For null pointer, examine key-swapping logic; for hashmap, check initialization parameters; for randomization, inspect the random number generation.

3. Apply Minimal Fixes

Implement the smallest change that fixes the root cause without altering unrelated behavior. For example, add null checks, correct hashmap capacity/load factor, or use a proper random seed.

4. Add Regression Tests

Write tests that specifically target each bug to ensure they are fixed and prevent future regressions. Include edge cases like null keys and boundary conditions.

5. Verify and Reflect

Run all tests to confirm fixes and no new issues. Discuss trade-offs of your fixes and potential improvements for robustness.

Key Points to Mention

Null pointer errors: likely due to missing null checks in key-swapping logic; fix by adding null guards or using Optional.
Hashmap misconfiguration: incorrect initial capacity or load factor causing performance issues or collisions; fix by setting appropriate values based on expected size.
Randomization bug: improper use of Random class (e.g., no seed, shared instance) leading to predictable or non-random behavior; fix by using ThreadLocalRandom or SecureRandom.
Testing: write unit tests for each bug, including edge cases, and use assertions to validate fixes.
Minimal fixes: avoid over-engineering; change only what's necessary to fix the bug.
Trade-offs: consider performance vs. correctness, and document any assumptions.

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