Meta·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Meta SWE interview with a debugging question around a maze renderer. Pretty focused, single problem, the kind of thing where you either see it fast or you spiral.

Questions Asked (1)

You're given a maze renderer that displays walls, open cells, a start, an end, and a computed path. A failing test shows the path symbol overwrites the start symbol. Find and fix the bug so the start symbol is always preserved, then verify with edge cases: path crosses the end cell, empty path, and start equals end.

Root Cause AnalysisAlgorithms & Data StructuresTechnical Trade-offs

Author's notes

The core bug is a priority issue in the rendering logic, path symbols are applied after start/end symbols so they clobber them.

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AI HintsAI Generated

Suggested Approach

Start by reproducing the failing test and tracing the rendering order to identify where the path symbol overwrites the start. Then fix the rendering logic to preserve the start symbol, and systematically verify with the specified edge cases to ensure correctness.

Pro tip: When fixing rendering order, consider using a priority-based approach where special cells (start/end) are drawn last, and always add regression tests for the edge cases to prevent future regressions.

1. Reproduce and Understand

Run the failing test and examine the maze renderer code to understand how symbols are placed and in what order. Identify the exact line where the path overwrites the start.

2. Identify Root Cause

Determine why the path symbol overwrites the start: likely the rendering order draws path after start, or the path includes the start cell and is drawn without checking for special cells.

3. Implement Fix

Modify the rendering logic to ensure the start symbol is always preserved. For example, render the path first, then overlay start and end symbols, or skip drawing path on start/end cells.

4. Verify with Edge Cases

Test the fix with the specified edge cases: path crosses the end cell (end should be preserved), empty path (start and end should still render), and start equals end (handle gracefully, perhaps show a combined symbol or prioritize start).

5. Add Regression Tests

Write unit tests for these edge cases to ensure the bug does not reappear and to document expected behavior.

Key Points to Mention

Rendering order and symbol priority (start/end should have higher priority than path)
Edge case handling: empty path, start equals end, path crossing end
Root cause analysis: tracing the code to find where the overwrite happens
Testing strategy: unit tests for edge cases and regression prevention
Code maintainability: separating concerns (path computation vs. rendering)
Trade-offs: performance vs. correctness when overlaying symbols

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