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Meta·Machine Learning Engineer·Onsite - Coding / Algorithms·Senior

SeniorPrefer not to say
Apr 2026

Summary

Meta MLE coding round where they handed me a broken maze-solver and asked me to find and fix two bugs. Pretty focused on reading existing code carefully rather than writing anything from scratch, which I wasn't fully expecting.

Questions Asked (1)

Q1

You're given a maze-solver codebase with two bugs: the start and end markers get overwritten by the path character in the rendered output, and the printed maze doesn't match the expected format. Find and fix both.

Algorithms & Data StructuresRoot Cause AnalysisTechnical Trade-offs
Author's notes

I spent way too long trying to rewrite the rendering logic before realizing the fix was just adding an if/else to give the special markers display priority.

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AI HintsAI Generated

Suggested Approach

Start by reproducing the bugs with a small test case and inspecting the code that renders the maze. Trace the logic for placing start/end markers and the path, then compare the output format against the expected specification. Fix the overwrite bug by ensuring markers are drawn after the path or by preserving them, and fix the format bug by adjusting the printing logic to match the expected delimiters and spacing.

Pro tip: Before diving into code, clarify the expected output format with the interviewer—this shows you prioritize requirements and avoid assumptions. Also, mention that you'd add a regression test to prevent these bugs from recurring.

1. Reproduce and Isolate

Run the maze solver on a simple input to reproduce both bugs. Isolate the rendering function and identify where start/end markers and path characters are written.

2. Diagnose the Overwrite Bug

Determine the order of operations: if the path is drawn after markers, it overwrites them. Check if markers are stored separately or if the path includes start/end cells.

3. Diagnose the Format Bug

Compare the printed maze to the expected format. Look for missing delimiters, incorrect spacing, or extra characters. Check if the format is hardcoded or derived from constants.

4. Implement Fixes

Fix the overwrite by rendering markers after the path or by excluding start/end from path drawing. Fix the format by adjusting the printing logic to match the expected specification.

5. Verify and Test

Re-run the solver and confirm both bugs are fixed. Add unit tests for rendering and format to prevent regressions.

Key Points to Mention

  • Order of rendering operations: path vs. markers
  • Separation of concerns: maze state vs. rendering logic
  • Importance of matching expected output format exactly
  • Writing regression tests for rendering and formatting
  • Using a debugger or print statements to trace execution
  • Considering edge cases: start/end on path, empty path, etc.

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