Cracking the Coding Interview Q1.6

Given an image represented by an NxN matrix, where each pixel in the image is 4 bytes, write a method to rotate the image by 90 degrees. Can you do this in place?

思路：从最晚层向内，一层一层交换。

    public static void rotate(int[][] matrix, int n) {
        for (int layer = 0; layer < n / 2; ++layer) {
            int first = layer;
            int last = n - 1 - layer;
            for(int i = first; i < last; ++i) {
                int offset = i - first;
                int top = matrix[first][i]; // save top

                // left -> top
                matrix[first][i] = matrix[last-offset][first];             

                // bottom -> left
                matrix[last-offset][first] = matrix[last][last - offset]; 

                // right -> bottom
                matrix[last][last - offset] = matrix[i][last]; 

                // top -> right
                matrix[i][last] = top; // right <- saved top
            }
        }
    }

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