Meta·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jun 2026

Summary

Meta coding interview with a debugging twist. Instead of writing something from scratch, they handed me broken code and asked me to find what was wrong with it.

Questions Asked (1)

You're given a maze-solving function that's already written. When it prints the path, it's including cells that shouldn't be walkable. Find the bug and fix it.

Algorithms & Data StructuresRoot Cause Analysis

Author's notes

Took me longer than I'd like to admit to stop reading the recursion logic and just focus on what was being printed.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the maze representation and the expected walkable cells, then trace the path output to identify which invalid cells are included. Systematically inspect the path construction and neighbor-checking logic, focusing on boundary conditions and the condition that determines if a cell is walkable. Once the bug is found, explain the fix and verify with edge cases.

Pro tip: Demonstrate a methodical debugging process: reproduce the issue with a minimal maze, add logging to see the path decisions, and consider both the algorithm's logic and the data structures used. This shows you can handle real-world debugging, not just theoretical problems.

1. Clarify the problem and expected behavior

Ask questions to confirm what constitutes a walkable cell (e.g., 0 vs 1, boundaries) and what the correct path should look like. This ensures you understand the bug's impact.

2. Reproduce and isolate the bug

Create a minimal maze that triggers the issue, run the function, and note the incorrect cells in the output. This helps narrow down where the bug might be.

3. Inspect the path construction and neighbor checks

Review how the path is built (e.g., backtracking, BFS) and how neighbors are validated. Look for off-by-one errors, incorrect boundary checks, or wrong conditions for walkability.

4. Identify and explain the root cause

Pinpoint the exact line(s) causing invalid cells to be added, such as a missing check for walls or an incorrect index. Explain why it happens.

5. Propose and verify the fix

Suggest a code change (e.g., add a condition, adjust indices) and mentally test it with edge cases like start/end at boundaries or no path. Confirm the fix resolves the issue without breaking other cases.

Key Points to Mention

Boundary conditions: ensure indices are within the maze dimensions.
Walkability check: verify the condition that determines if a cell is passable (e.g., value == 0).
Path construction logic: check if the algorithm adds cells before validating them.
Data structures: consider if the maze is represented as a grid of characters or integers and if the check matches.
Off-by-one errors: common in loops and index calculations.
Testing: use edge cases like a 1x1 maze, start/end at corners, and mazes with no solution.

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