Q4: How do you rotate an N x N 2D matrix 90 degrees clockwise in-place?

Rotating a matrix 90 degrees clockwise in-place involves two steps:

Transpose the matrix: Swap matrix[i][j] with matrix[j][i] for all $i < j$.
Reverse each row: Swap elements from outer columns inward (matrix[i][left] with matrix[i][right]).

Matrix 90 Degree Rotation

public class RotateMatrix {
    public static void rotate(int[][] matrix) {
        int n = matrix.length;
        // Step 1: Transpose
        for (int i = 0; i < n; i++) {
            for (int j = i + 1; j < n; j++) {
                int temp = matrix[i][j];
                matrix[i][j] = matrix[j][i];
                matrix[j][i] = temp;
            }
        }
        // Step 2: Reverse rows
        for (int i = 0; i < n; i++) {
            int left = 0, right = n - 1;
            while (left < right) {
                int temp = matrix[i][left];
                matrix[i][left] = matrix[i][right];
                matrix[i][right] = temp;
                left++; right--;
            }
        }
    }
}

💡 Transposes the grid matrix across its main diagonal and flips rows horizontally.

Key Takeaways

Step 1: Transpose matrix along the main diagonal.
Step 2: Reverse elements horizontally in each row.

Knowledge Check

How would you rotate an N x N matrix 90 degrees COUNTER-clockwise in-place?

Q4: How do you rotate an N x N 2D matrix 90 degrees clockwise in-place?

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