leetcode -- 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?

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 void rotate(int[][] matrix) {
        // Start typing your Java solution below
        // DO NOT write main() function
        int len = matrix.length;
        if(len == 0)
            return;
        
        for(int i = 0; i < len - 1; i++){
            for(int j = 0; j < len -i; j++){
                swap(matrix, i, j, len - 1 - j, len - 1 -i);
            }
        }
        
        
        for(int i = 0; i < len / 2; i++){
            for(int j = 0; j < len; j++){
                swap(matrix, i, j, len - 1 -i, j);
            }
        }
    }
    
    public void swap(int[][] matrix, int i, int j, int m, int n){
        int tmp = matrix[i][j];
        matrix[i][j] = matrix[m][n];
        matrix[m][n] = tmp;
    }