rotate image(旋转数组)

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]
]

这题就相当于找规律。。
规律:将第一行和最后一行交换，第二行和倒数第二行交换，。。。然后将关于对角线对称的元素交换。

代码如下：注意二维数组的长度表示。

class Solution {
    public void rotate(int[][] matrix) {
        //第一步：将第一行和最后一行交换，第二行和倒数第二行交换。。
        for(int i=0;i<matrix.length/2;i++)
            for(int j=0;j<matrix[0].length;j++){
                int tmp=matrix[i][j];
                matrix[i][j]=matrix[matrix.length-1-i][j];
                matrix[matrix.length-1-i][j]=tmp;
            }

        //第二步：交换关于对角线对称的元素
        for(int i=0;i<matrix.length;i++)
            for(int j=i+1;j<matrix[0].length;j++){
                int tem=matrix[i][j];
                matrix[i][j]=matrix[j][i];
                matrix[j][i]=tem;
            }
    }
}