Amazon-Rotate Image (Medium)

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.

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]
]

Example 2:

Given input matrix =
[
  [ 5, 1, 9,11],
  [ 2, 4, 8,10],
  [13, 3, 6, 7],
  [15,14,12,16]
], 

rotate the input matrix in-place such that it becomes:
[
  [15,13, 2, 5],
  [14, 3, 4, 1],
  [12, 6, 8, 9],
  [16, 7,10,11]
]

Solution:

    public void rotate(int[][] matrix) {
        
        if(matrix==null || matrix.length==0) return ;
        
        int n=matrix.length;
        for(int i=0;i<n;i++){
            for(int j=i;j<n;j++){
                int tmp=matrix[i][j];
                matrix[i][j]=matrix[j][i];
                matrix[j][i]=tmp;
            }
        }
        
        for(int i=0;i<n/2;i++){
            for(int j=0;j<n;j++){
                int tmp=matrix[j][i];
                matrix[j][i]=matrix[j][n-1-i];
                matrix[j][n-1-i]=tmp;
            }
        }
        
        return;
    }

时间复杂度：O(n^2)
空间复杂度：O(1)

规律题，不太好找到。右旋90°等同于矩阵先沿对角线互换再沿左右对换。右旋180°，可以进行两次90°旋转，也可以上下对换再左右对换。以此类推。
题目中假定是n*n矩阵，建议还是做一下输入合法性判断。