Rotate Image

You are given an n x n 2D matrix representing an image.

Rotate the image by 90 degrees (clockwise).

Follow up:
Could you do this in-place?

参考：

水中的鱼: [LeetCode] Rotate Image 解题报告

[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。

 

public class Solution {
    public void rotate(int[][] matrix) {
        if(matrix.length == 0)
            return;
        
        int n = matrix.length;
        // 注意反转矩阵反转矩阵的一半即可，全部反转等于没反转
        for(int i = 0; i<n-1; i++){
            for(int j =0; j < n-i; j++){
                swap(matrix, i, j, n-1-j, n-1-i);
            }
        }
        
        for(int i = 0; i < n/2; i++){
            for(int j =0; j < n; j++){
                swap(matrix, i, j, n-1-i, j);
            }
        }
    }
    
    public void swap(int[][] matrix, int i, int j, int p, int q){
        int tmp = matrix[i][j];
        matrix[i][j] = matrix[p][q];
        matrix[p][q] = tmp;
        
    }
}