Meta·Software Engineer·Technical Phone Screen·Senior

Senior

Apr 2026

Summary

Meta SWE interview, technical phone screen style. One meaty coding question that started simple but got extended with a constraint mid-way. Felt like a design-your-way-out-of-a-corner kind of problem.

Questions Asked (1)

You have a BFS-based maze solver on an m x n grid. Some cells only allow horizontal movement (no entering or leaving them vertically). How do you update the neighbor-generation logic to enforce this, how do you represent those constraints in the data structure, and how would you test it?

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

Started fine with the baseline BFS, felt confident.

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AI HintsAI Generated

Suggested Approach

Explain that you need to augment the grid representation to store per-cell movement constraints, then modify the BFS neighbor generation to check both the current cell's and the neighbor's constraints before allowing a move. Emphasize that the core BFS logic remains unchanged, and testing should cover edge cases like corners, isolated cells, and unreachable targets.

Pro tip: Mention that you can precompute allowed moves for each cell to avoid repeated condition checks, and discuss how this approach scales to other directional constraints (e.g., one-way doors) without changing the BFS algorithm.

1. Represent constraints in the grid

Augment the grid data structure to store movement permissions per cell, such as a bitmask or enum indicating allowed directions (e.g., horizontal-only, vertical-only, all).

2. Modify neighbor generation

In the BFS neighbor loop, for each candidate direction, check that the current cell allows exiting in that direction and the neighbor cell allows entering from the opposite direction.

3. Handle edge cases and boundaries

Ensure boundary checks are still performed, and consider special cases like start/end cells with constraints, or cells that are completely blocked.

4. Test with targeted scenarios

Design unit tests for simple grids (e.g., 2x2, 3x3) with known paths, including cases where horizontal-only cells force detours, and verify BFS finds the shortest valid path or correctly reports unreachable.

5. Discuss trade-offs and optimizations

Mention that precomputing allowed moves per cell can improve performance, and that the approach generalizes to other directional constraints without altering BFS's time complexity.

Key Points to Mention

Use a bitmask or enum to represent allowed movement directions per cell (e.g., 1 for left, 2 for right, 4 for up, 8 for down).
Neighbor generation must check both the current cell's exit permission and the neighbor's entry permission.
BFS still explores level by level; only the adjacency definition changes.
Testing should include grids where horizontal-only cells create bottlenecks or force longer paths.
Consider precomputing a list of valid moves for each cell to avoid redundant checks during BFS.
The solution should handle start and end cells that may have constraints, and cells that are completely isolated.

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