You are given annxn2D matrix representing an image.

Rotate the image by 90 degrees (clockwise).

Note:
You have to rotate the imagein-place, which means you have to modify the input 2D matrix directly.DO NOTallocate another 2D matrix and do the rotation.

Example 1:

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]
]

[解题思路]

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。

void rotate(vector<vector<int> > &matrix) {  
      // Start typing your C/C++ solution below  
      // DO NOT write int main() function  
      int len = matrix[0].size();  
      for(int i =0; i<len-1; i++)  
      {  
        for(int j=0;j<len-i;j++)  
        {  
          swap(matrix[i][j], matrix[len-1-j][len-1-i]);  
        }  
      }  
      for(int i =0; i<len/2; i++)  
      {  
        for(int j=0;j<len;j++)  
        {  
          swap(matrix[i][j], matrix[len-i-1][j]);  
        }  
      }  
    }  
    void swap(int& a1, int&a2)  
    {  
      int temp = a1;  
      a1=a2;  
      a2=temp;  
    }