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 :
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]
]
方案：
首先转置，然后每行的数字翻转：
1  2  3　　　 　　 1  4  7　　　　　  7  4  1
4  5  6　　-->　　 2  5  8　　 -->   8  5  2　　
7  8  9 　　　 　　3  6  9　　　　   9  6  3