Meta·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jun 2026Remote

Summary

Meta SWE coding round with a grid/island problem. Pretty standard stuff but the follow-up pressure to optimize was real.

Questions Asked (1)

Given an n x n binary matrix, you can flip at most one 0 to a 1. What is the maximum possible island area you can achieve? Islands are groups of 1s connected in four directions.

Algorithms & Data Structures

Author's notes

My first instinct was just BFS every cell and try flipping each zero, which works but is way too slow for a 500x500 grid.

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AI HintsAI Generated

Suggested Approach

First, identify all existing islands and label each cell with its island ID and area. Then, for each 0 cell, compute the sum of areas of distinct neighboring islands (up to 4) and take the maximum; also consider the case of no flip (max existing island area).

Pro tip: Mention edge cases upfront: all 1s (return n*n), all 0s (return 1), and single row/column. This shows thoroughness and often impresses interviewers.

1. Clarify and handle edge cases

Confirm the problem constraints and discuss edge cases like all 1s, all 0s, and 1x1 matrix. This ensures you cover all scenarios.

2. Identify existing islands

Use BFS/DFS or Union-Find to label each island with a unique ID and compute its area. Store the area in a map or array.

3. Evaluate each 0 cell

For each 0, collect the distinct island IDs of its 4 neighbors, sum their areas, and add 1 for the flipped cell. Track the maximum.

4. Consider no flip

Also consider the maximum island area without flipping any 0, as flipping might not always increase the area (e.g., all 1s).

5. Return the maximum

Return the larger of the maximum area from flipping a 0 and the maximum existing island area.

Key Points to Mention

Use BFS/DFS or Union-Find to label islands and compute areas efficiently.
For each 0, only consider distinct neighboring islands to avoid double-counting.
Time complexity: O(n^2) with BFS/DFS, O(n^2 α(n)) with Union-Find; space complexity: O(n^2).
Edge cases: all 1s (return n*n), all 0s (return 1), single row/column.
Optimization: precompute island areas and store in a hash map for O(1) lookup.
Alternative approach: Union-Find with path compression and union by rank.

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