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?

Solution: 基本思路是把图片分为行数/2层，然后一层层进行旋转，每一层有上下左右四个列，然后目标就是把上列放到右列，右列放到下列，下列放到左列，左列放回上列，中间保存一个临时变量即可。因为每个元素搬运的次数不会超过一次，时间复杂度是O(n^2)，空间复杂度是O(1)。

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