48. Rotate Image

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

Rotate the image by 90 degrees (clockwise).

Note:
You have to rotate the image in-place, which means you have to modify the input 2D matrix directly.DO NOT allocate another 2D matrix and do the rotation.

\Given input matrix =
[
  [1,2,3],
  [4,5,6],
  [7,8,9]
],

rotate the input matrix in-place such that it becomes:
[
  [7,4,1],
  [8,5,2],
  [9,6,3]
]

这道题需要有几何方面的知识（我认为==，毕竟初中数学已经忘完了）。旋转90度等于先镜像一次，然后左右对称交换。如：

1  2  3             1   4  7      7   4   1 

4  5  6             2   5  8      8   5   2

7  8  9             3   6   9     9   6   3

对应代码就是：

public void rotate(int[][] matrix) {        int n = matrix.length;        for(int i=0;i<n;i++){

//镜像交换

//注意这里j应该是从i+1开始 否则就是不变（好好考虑一下）            for(int j=i+1;j<n;j++){                int temp = matrix[i][j];                matrix[i][j] = matrix[j][i];                matrix[j][i] = temp;            }        }        for(int i=0;i<n;i++){

        //这里是每一行做交换            for(int j=0;j<n/2;j++){                int temp = matrix[i][j];                matrix[i][j] = matrix[i][n-1-j];                matrix[i][n-1-j] = temp;            }        }    }