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Microsoft·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
Jun 2026

Summary

Microsoft SWE coding round, one algorithmic question on grid islands. Pretty focused session, no fluff.

Questions Asked (1)

Q1

Given an n x n binary grid, you can flip at most one 0 to a 1. What is the largest 4-connected island of 1s you can form after that flip?

Algorithms & Data Structures
Author's notes

This one took me a minute to get past the brute force instinct.

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AI HintsAI Generated

Suggested Approach

First, identify all existing islands of 1s and label them with unique IDs while recording their sizes. Then, for each 0 cell, sum the sizes of distinct neighboring islands (up, down, left, right) and add 1 for the flipped cell; track the maximum. Also consider the case where no flip is made, returning the largest existing island size.

Pro tip: Mention edge cases upfront: grid with no 1s (answer 1 if n>0), grid full of 1s (answer n*n), and ensure you deduplicate island IDs when a 0 touches the same island from multiple sides.

1. Clarify and handle edge cases

Confirm grid dimensions, connectivity (4-directional), and that flipping is optional. Discuss edge cases like all 0s, all 1s, and n=1.

2. Label islands and compute sizes

Use BFS/DFS to assign a unique ID to each island of 1s and store its size in a hash map. This takes O(n^2) time.

3. Evaluate each 0 cell

For each 0, collect the IDs of its 4-neighbors, sum the sizes of distinct islands, add 1, and update the maximum.

4. Compare with no-flip scenario

Track the maximum island size found during labeling; the final answer is the max of that and the best flip result.

5. Analyze complexity and optimize

State O(n^2) time and O(n^2) space. Mention that the grid can be modified in-place for labeling to save space, or use a visited matrix.

Key Points to Mention

  • Use BFS/DFS to label islands and compute sizes efficiently.
  • Deduplicate island IDs when a 0 cell touches the same island from multiple directions.
  • Consider the case where flipping is not beneficial (e.g., grid already has a large island).
  • Handle edge cases: no 1s, all 1s, and n=1.
  • Time complexity: O(n^2) with a single pass for labeling and another pass for evaluating 0s.
  • Space complexity: O(n^2) for the visited/label matrix, but can be optimized to O(n^2) in-place or O(n) with union-find.

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