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?
题意：矩阵旋转。
思路：每次旋转的时候都一次性旋转4个数。

class Solution {
public:
    void rotate(vector<vector<int> > &matrix) {
        int n = matrix.size() - 1;
        for (int i = 0; i < n; i++) {
            for (int j = i; j < n-i; j++) {
                int x = i, y = j;
                int pre = matrix[x][y];
                for (int k = 0; k < 4; k++) {
                    int next = matrix[y][n-x];
                    matrix[y][n-x] = pre;
                    int tmp = x;
                    x = y;
                    y = n - tmp;
                    pre = next; 
                }
            }
        } 
    }
};