Amazon·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Did a technical phone screen for a Software Engineer role at Amazon. One coding question, interviewer was pretty relaxed about the whole thing.

Questions Asked (1)

Given an N x N matrix, rotate it 90 degrees in place.

Algorithms & Data Structures

Author's notes

Classic problem but I always second-guess myself on whether to transpose first or do the reverse.

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AI HintsAI Generated

Suggested Approach

Clarify the rotation direction (clockwise or counterclockwise) and confirm the matrix is square. The optimal in-place solution is to first transpose the matrix and then reverse each row (for clockwise rotation) or reverse each column (for counterclockwise). Explain the time and space complexity and handle edge cases like 1x1 or empty matrix.

Pro tip: Mention that the transpose-and-reverse method is equivalent to a 4-way swap of elements in cycles, but the former is simpler to implement and less error-prone. Also, discuss how the approach can be adapted for non-square matrices if needed.

1. Clarify requirements

Ask whether the rotation is clockwise or counterclockwise, and confirm the matrix is square (N x N). Also check if in-place is mandatory or if extra space is allowed.

2. Choose approach

Decide between the transpose-and-reverse method (simpler) or the layer-by-layer 4-way swap (more direct). Explain the trade-offs.

3. Implement transpose

For clockwise rotation, swap matrix[i][j] with matrix[j][i] for all i < j. This transposes the matrix in place.

4. Reverse rows or columns

For clockwise, reverse each row; for counterclockwise, reverse each column. This completes the rotation.

5. Analyze complexity and edge cases

State O(N^2) time and O(1) space. Test with 1x1, 2x2, and empty matrix. Mention that the method works for any N.

Key Points to Mention

Time complexity: O(N^2) because every element is visited once during transpose and once during reversal.
Space complexity: O(1) extra space since the rotation is done in place.
The transpose-and-reverse method is equivalent to rotating the matrix 90 degrees clockwise.
For counterclockwise rotation, reverse columns after transpose instead of rows.
Edge cases: empty matrix (N=0), 1x1 matrix, and matrices with duplicate values.
Alternative approach: rotate in layers by swapping four elements at a time, which is more complex but avoids two passes.

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