Microsoft·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jun 2026

Summary

Microsoft SWE interview with a matrix manipulation problem that had more edge cases than I expected. The in-place constraint is what made it actually hard.

Questions Asked (1)

Given a large matrix and a sub-matrix defined by its corner coordinates, move the sub-matrix to a new position within the larger matrix. You must do this in-place without allocating a second matrix of the same size. How do you handle overlapping source and destination regions, and how do you order your reads and writes to avoid corrupting data?

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I jumped straight to the non-overlapping case and coded it up fine, then the interviewer asked what happens when source and destination overlap.

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AI HintsAI Generated

Suggested Approach

First, clarify the problem constraints and define the sub-matrix and target position. Then, analyze the overlap between source and destination regions to determine the safe order of copying elements (e.g., top-left to bottom-right or vice versa). Finally, implement an in-place move using temporary storage for a single row or element, and discuss trade-offs like time complexity and edge cases.

Pro tip: Mention that you can avoid temporary storage for the entire sub-matrix by using a single temporary variable and carefully ordering the copies, but be prepared to discuss the trade-off between simplicity and performance.

1. Clarify the problem

Restate the problem to ensure understanding: moving a sub-matrix defined by its top-left and bottom-right coordinates to a new top-left position within the same matrix, in-place. Ask about constraints like matrix size, data type, and whether the sub-matrix must be preserved exactly.

2. Analyze overlap

Determine the relative positions of the source and destination rectangles. Identify if they overlap and in which direction (e.g., destination is above-left, below-right, etc.). This dictates the safe order of copying to avoid overwriting unread source data.

3. Choose copy order

Based on the overlap analysis, decide whether to copy elements from top-left to bottom-right or from bottom-right to top-left. For example, if the destination is below and to the right of the source, copy from bottom-right to top-left to avoid overwriting.

4. Implement in-place move

Iterate over the sub-matrix in the chosen order, using a temporary variable to hold each element before writing it to the destination. Ensure that each source element is read before it is overwritten by a previous write.

5. Discuss complexity and edge cases

State the time complexity O(rows * cols) and space complexity O(1). Mention edge cases: sub-matrix at matrix boundaries, non-overlapping regions, and when source and destination are identical.

Key Points to Mention

Overlap detection: compare source and destination rectangles to determine if they intersect.
Copy direction: if destination is above-left of source, copy top-left to bottom-right; if below-right, copy bottom-right to top-left.
Temporary storage: use a single variable or a temporary row to hold data during the move.
In-place constraint: no additional matrix of the same size; only O(1) extra space.
Time complexity: O(m*n) where m and n are dimensions of the sub-matrix.
Edge cases: sub-matrix at edges, non-overlapping move, and identical source/destination.

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