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?

思路：先沿着主对角线交换矩阵内的数字，然后沿中间列交换。

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

[LeetCode] Rotate Image