Rotate Image

一、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?

二、思路

矩阵旋转90度。原理如下：先上下翻转，再交换对角元素即可。

 * 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

三、代码

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

在评论区看到另外一种思路：先左右翻转，然后交换对角元素。

* 1 2 3     3 2 1     3 6 9
 * 4 5 6  => 6 5 4  => 2 5 8
 * 7 8 9     9 8 7     1 4 7

void anti_rotate(vector<vector<int> > &matrix) {
    for (auto vi : matrix) reverse(vi.begin(), vi.end());
    for (int i = 0; i < matrix.size(); ++i) {
        for (int j = i + 1; j < matrix[i].size(); ++j)
            swap(matrix[i][j], matrix[j][i]);
    }
}