leetcode 48. Rotate Image

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]
]

Example 2:

Given input matrix =
[
  [ 5, 1, 9,11],
  [ 2, 4, 8,10],
  [13, 3, 6, 7],
  [15,14,12,16]
], 

rotate the input matrix in-place such that it becomes:
[
  [15,13, 2, 5],
  [14, 3, 4, 1],
  [12, 6, 8, 9],
  [16, 7,10,11]
]

模拟一下，先看四个角，每次交换两个，交换三次就可以了。对于左上角向下走一个单位找到a，左下角向右移动一个单位b，右下角向上移动c，右上角向左移动d。交换(a,b)(b,c),(c,d)....
具体见代码:

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