30.Rotate Image（矩阵旋转）

Level：

  Medium

题目描述：

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]
]

思路分析：

  由于题目要求只能在原地进行旋转，那么我们不能使用额外的辅助空间，所以先进行matrix[i] [j]与matrix[j] [i]进行交换，然后matrix[i] [j]和matrix[i] [len-j]交换就能得到结果。

代码;

public class Solution{
    public void rotate(int [][]matrix){
        for(int i=0;i<matrix.length;i++){
            for(int j=i;j<matrix[0].length;j++){
                int temp=matrix[i][j];
                matrix[i][j]=matrix[j][i];
                matrix[j][i]=temp;
            }
        }
        for(int i=0;i<matrix.length;i++){
            for(int j=0;j<matrix[0].length/2;j++){
                int temp1=matrix[i][j];
                matrix[i][j]=matrix[i][matrix[0].length-1-j];
                matrix[i][matrix[0].length-1-j]=temp1;
            }
        }
    }
}