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?

根据一个点坐标计算出其他三个点的位置。

可以依据，每个点到外边框的对应距离相等，推算坐标。

class Solution {
public:
    void rotate(vector<vector<int> > &matrix) {
        const int n = matrix.size();
        for (int i=0; i<n/2; i++) {
                for (int j=i; j<n-i-1; j++) {
                        const int bak = matrix[i][j];
                        matrix[i][j] = matrix[n-1-j][i];
                        matrix[n-1-j][i] = matrix[n-1-i][n-1-j];
                        matrix[n-1-i][n-1-j] = matrix[j][n-1-i];
                        matrix[j][n-1-i] = bak;
                }
        }
    }
};

Rotate Image -- leetcode