Meta·Machine Learning Engineer·Technical Phone Screen·Senior

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Jun 2026Remote

Summary

Meta coding round for an MLE role, one problem the whole session. They gave me an existing maze traversal implementation and asked me to extend it with directional cell constraints. Felt manageable at first but the edge cases piled up fast.

Questions Asked (1)

You're given an existing maze traversal solution with a move(from_cell, to_cell) method. The maze now includes directional cells ('>' for left-to-right only, '<' for right-to-left only). Modify only the move method to enforce these directional constraints, and walk through edge cases like starting on a directional cell, two adjacent conflicting directional cells, and how this change interacts with the BFS/DFS caller.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The base problem clicked pretty fast, checking the direction of movement against the cell marker is just arithmetic on coordinates.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the move method's contract and the maze's cell representation, then design a minimal change that checks directional constraints before allowing a move. Walk through the specified edge cases to validate the logic, and explain how the caller (BFS/DFS) remains unaffected because it only relies on move's boolean return.

Pro tip: Emphasize that the move method should be a pure function with no side effects, and that directional constraints are local to the edge, not the cell—this shows you understand separation of concerns and testability.

1. Clarify the problem and constraints

Restate the move method's signature and expected behavior, and confirm that only move can be modified. Identify how directional cells are represented and what 'from' and 'to' mean in terms of coordinates.

2. Design the directional check

Determine the direction of the move (e.g., left-to-right or right-to-left) based on coordinates. Check if the from_cell or to_cell has a directional constraint that conflicts with the move direction.

3. Handle edge cases explicitly

Walk through: starting on a directional cell (does it restrict leaving?), two adjacent conflicting directional cells (e.g., '>' then '<'), and moves that are not horizontal (if applicable). Decide on behavior for each.

4. Integrate with BFS/DFS caller

Explain that the caller only uses move's boolean return to decide whether to traverse an edge, so no changes are needed. Highlight that this preserves the algorithm's correctness and complexity.

5. Test and validate

Propose unit tests for each edge case and for normal moves. Discuss potential pitfalls like off-by-one errors or misinterpretation of direction.

Key Points to Mention

The move method should return a boolean indicating whether the move is allowed.
Directional constraints apply to the edge between cells, not the cells themselves.
Starting on a directional cell: the constraint only applies when moving in the restricted direction; moving opposite or perpendicular may be allowed.
Two adjacent conflicting directional cells: the move is blocked if either cell's constraint forbids the direction.
BFS/DFS caller remains unchanged because it only checks move's return value.
Time complexity of move should remain O(1) to not affect overall algorithm performance.

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