LeetCode算法题之Rotate Image

问题描述：
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?
要求不能另开空间，一开始理解错了，后来画图明白了
解题思路：
首先以对角线为轴对称交换，再以中间列（如果有的话）为轴对称交换
a b c d
e f g h
i j k l
m n o p

a e i m
b f j n
c g k o
d h l p

m i e a
n j f b
o k g c
p l h d

class Solution {
public:
    void rotate(vector<vector<int> > &matrix) {
        if (matrix.empty()) 
            return;
        int n = matrix.size(), 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;
            }
        }
        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;
            }
        }
        return;
    }
};