Leetcode: Rotate Image

题目：
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?

思路分析：
最笨的方法，重新开辟一个矩阵空间，做旋转。（题目要求最好能就地旋转）
更好的方法：先将矩阵上下对折，然后再沿对角线对折。
例如：

1  2  3      7  8  9      7  4  1
4  5  6  ->  4  5  6  ->  8  5  2
7  8  9      1  2  3      9  6  3

OK，开始写代码！
C++参考代码：

class Solution
{
public:
    void rotate(vector<vector<int> > &matrix)
    {
        if (matrix.empty()) return;
        int rows = int(matrix.size());
        int cell = 0;
        //上下对折
        for (int i = 0; i < rows / 2; ++i)
        {
            for (int j = 0; j < rows; ++j)
            {
                cell = matrix[i][j];
                matrix[i][j] = matrix[rows - i - 1][j];
                matrix[rows - i - 1][j] = cell;
            }
        }
        //对角线对折
        for (int i = 0; i < rows; ++i)
        {
            for (int j = 0; j < i; ++j)
            {
                cell = matrix[i][j];
                matrix[i][j] = matrix[j][i];
                matrix[j][i] = cell;
            }
        }
    }
};