Meta·Software Engineer·Onsite - Coding / Algorithms·Intermediate

Intermediate

Jun 2026

Summary

Meta SWE coding round with a maze manipulation problem. Pretty focused on getting the edge cases right around preserving special characters on the path.

Questions Asked (1)

Given a 2D character grid representing a maze with start (S), end (E), walls (#), and open cells (.), and a precomputed path through it, modify the grid so that open cells on the path become *, but S and E stay as-is. All other cells remain unchanged.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The naive solution just stamps * on everything in the path list, which breaks S and E.

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AI HintsAI Generated

Suggested Approach

Clarify that the precomputed path is given as a list of coordinates, then iterate through each coordinate and update the grid cell to '*' only if it is not 'S' or 'E'. Emphasize that this is an in-place modification with O(P) time where P is path length, and discuss edge cases like overlapping start/end or invalid paths.

Pro tip: Mention that you would avoid mutating the original grid if the caller might need it later, and instead return a new grid or document the in-place behavior—this shows awareness of side effects and API design.

1. Clarify inputs and assumptions

Confirm that the path is a sequence of (row, col) coordinates and that the grid is mutable. Ask whether the path includes S and E, and whether it's guaranteed to be valid.

2. Iterate through the path

Loop over each coordinate in the precomputed path. For each, check the current cell value.

3. Conditionally update cells

If the cell is not 'S' or 'E', set it to '*'. Otherwise, leave it unchanged to preserve start and end markers.

4. Handle edge cases and validate

Consider empty path, path with invalid coordinates, or path that includes walls. Decide whether to throw an error or skip invalid cells.

5. Analyze complexity and trade-offs

State time complexity O(P) and space O(1) for in-place modification. Discuss whether to mutate in-place or return a new grid based on requirements.

Key Points to Mention

In-place modification vs. creating a copy: trade-offs of side effects and memory.
Time and space complexity: O(P) time, O(1) extra space for in-place.
Edge cases: empty path, path containing S/E, invalid coordinates, walls on path.
Preserving S and E: explicit check before overwriting.
Path representation: assume list of (row, col) tuples; clarify if not.
Testing: verify with a small example and consider unit tests for boundary conditions.

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