[Leetcode] 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?

用了最笨的办法。就地转的话应该是一圈一圈的转吧，有空再写写。

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

第二次刷leetcode发现好多题第一次做时就是A掉就不管了，其实还有很多东西可以继续挖掘，所以！要多刷几遍啊！原地转的话，这里用到一个小技巧，把一张纸上下折，再按对角线对折，那么它就转了90度了有木有，所以这里就用了这个技巧。

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