48. Rotate Image 

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class Solution {
    public void rotate(int[][] matrix) {
        // 1, 2, 3      7, 8, 9      7, 4, 1
        // 4, 5, 6  ->  4, 5, 6  ->  8, 5, 2
        // 7, 8, 9      1, 2, 3      9, 6, 3

        int n = matrix.length;
        for (int i = 0; i < n / 2; i++) {
            for (int j = 0; j < n; j++) {
                int tmp = matrix[i][j];
                matrix[i][j] = matrix[n - i - 1][j];
                matrix[n - i - 1][j] = tmp;
            }
        }

        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                if (i <= j) {
                    break;
                }

                // swap: (0, 1), (0, 2), (1, 2)
                int tmp = matrix[i][j];
                matrix[i][j] = matrix[j][i];
                matrix[j][i] = tmp;
            }
        }
    }
}

References

48. Rotate Image
面试题 01.07. 旋转矩阵