leetcode 48. Rotate Image

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：

题目要求不能创建新的矩阵。

step1:将矩阵按行进行翻转。

step2:将step1的结果的对角线进行交换。

实现：

class Solution {
    public void rotate(int[][] matrix) {
        //题目要求不能创建新的矩阵
        //先对矩阵的行进行一次翻转，再将对角线两边的元素转置
        int s=0;
        int e=matrix.length-1;
        //先对行元素进行翻转
        while(s<e){
            int[] temp=matrix[s];
            matrix[s]=matrix[e];
            matrix[e]=temp;
            s++;
            e--;
        }
        //对角线上的元素进行转至
        for(int i=0;i<matrix.length;i++){
            for(int j=i+1;j<matrix[i].length;j++){
                int temp=matrix[i][j];
                matrix[i][j]=matrix[j][i];
                matrix[j][i]=temp;
            }
        }
    }
}