How would you rotate a matrix by 90 degrees in place?
A coding challenge testing your spatial reasoning and ability to manipulate 2D arrays with strict memory constraints.
Why Interviewers Ask This
This problem evaluates your ability to think geometrically and implement in-place transformations without extra space. It tests your grasp of array indexing and nested loops. Interviewers often use this to distinguish candidates who can optimize space complexity effectively.
How to Answer This Question
Explain the two-step approach: transpose the matrix followed by reversing each row. Alternatively, describe rotating layers from outside in. Clearly define the index mapping for rotation. Discuss time complexity O(N^2) and space complexity O(1).
Key Points to Cover
- Transpose then reverse
- In-place transformation
- Index mapping logic
- O(1) space
Sample Answer
To rotate a matrix 90 degrees clockwise in place, I would first transpose the matrix by swapping elements across the main diagonal. Then, I would reverse each row of the transposed matrix. This two-step process achieves…
Common Mistakes to Avoid
- Creating a new matrix
- Incorrect index calculation
- Confusing clockwise vs counter-clockwise
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