leetcode -- Rotate Image

You are given an n x n 2D matrix representing an image.

Rotate the image by 90 degrees (clockwise).

Follow up:
Could you do this in-place?

如下图，首先沿逆对角线翻转一次，然后按x轴中线翻转一次。

 1 public void rotate(int[][] matrix) {

 2         // Start typing your Java solution below

 3         // DO NOT write main() function

 4         int len = matrix.length;

 5         if(len == 0)

 6             return;

 7         

 8         for(int i = 0; i < len - 1; i++){

 9             for(int j = 0; j < len -i; j++){

10                 swap(matrix, i, j, len - 1 - j, len - 1 -i);

11             }

12         }

13         

14         

15         for(int i = 0; i < len / 2; i++){

16             for(int j = 0; j < len; j++){

17                 swap(matrix, i, j, len - 1 -i, j);

18             }

19         }

20     }

21     

22     public void swap(int[][] matrix, int i, int j, int m, int n){

23         int tmp = matrix[i][j];

24         matrix[i][j] = matrix[m][n];

25         matrix[m][n] = tmp;

26     }