原题地址：https://oj.leetcode.com/problems/rotate-image/
题意：
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?
解题思路：先将矩阵转置，然后将矩阵的每一行翻转，就可以得到所要求的矩阵了。
代码：
class Solution:
# @param matrix, a list of lists of integers
# @return a list of lists of integers
def rotate(self, matrix):
n = len(matrix)
for i in range(n):
for j in range(i+1, n):
matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]
for i in range(n):
matrix[i].reverse()
return matrix