48. Rotate Image

https://leetcode.com/problems/rotate-image/description/

You are given an n x n 2D matrix representing an image.

Rotate the image by 90 degrees (clockwise).

Note:
You have to rotate the image in-place, which means you have to modify the input 2D matrix directly. DO NOT allocate another 2D matrix and do the rotation.

Example 1:

Given input matrix = 
[
  [1,2,3],
  [4,5,6],
  [7,8,9]
],

rotate the input matrix in-place such that it becomes:
[
  [7,4,1],
  [8,5,2],
  [9,6,3]
]

正向旋转90度

先求出按副对角线对称的矩阵 再上下翻转。

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