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1：首先沿着副对角线翻转一次，然后沿着水平中线翻转一次，就可以得到所要求的矩阵了。时间复杂度O(n^2)，空间复杂度O(1)

class Solution:
    # @param matrix, a list of lists of integers
    # @return a list of lists of integers
    def rotate(self, matrix):
        #思路1，时间复杂度O(n^2)，空间复杂度O(1)
        n = len(matrix)
        for i in range(n):
            for j in range(n-i): #沿着副对角线反转
                matrix[i][j], matrix[n-1-j][n-1-i] = matrix[n-1-j][n-1-i], matrix[i][j]
        for i in range(n/2):     #沿着水平中线反转
            for j in range(n):
                matrix[i][j], matrix[n - 1 - i][j] = matrix[n - 1 - i][j], matrix[i][j]
        return matrix

解题思路2：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。

reference: http://www.cnblogs.com/zuoyuan/p/3772978.html

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]  #先将矩阵转置
        for i in range(n):
            matrix[i].reverse()  #然后将矩阵的每一行翻转
        return matrix