Rotate Image

Q:

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?

A:

假设为n*n矩阵，则从外至内有n/2个圈需要旋转，每个圈旋转的时候是有固定的规律的，在纸上写一写应该就能发现，matrix[i][j] = matrix[n - j - 1][i]，代码就很easy了。时间复杂度o(n^2)，空间复杂度o(1),ok!

class Solution {
public:
    void rotate(vector<vector<int> > &matrix) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int len = matrix.size();
        if (len <= 1) return;
        for (int i = 0; i < (len + 1)/2; ++i) {
            for (int j = i; j < len - i - 1; ++j) {
                int r = i;
                int c = j;
                int org = matrix[r][c];
                for (int k = 0; k < 3; ++k) {
                    matrix[r][c] = matrix[len - 1 - c][r];
                    int tmp = r; 
                    r = len - 1 - c;
                    c = tmp;
                }
                matrix[r][c] = org;
            }
        }
    }
};