Given a 32-bit signed integer, reverse digits of an integer.

Example 1:                               Input: 123                       Output: 321

Example 2:                               Input: -123                      Output: -321

Example 3:                               Input: 120                        Output: 21

Note:  Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range: [−2³¹,  2³¹ − 1]. For the purpose of this problem, assume that your function returns 0 when the reversed integer overflows.

思路：对于负数设置一个标志位，然后将其转化成正整数，再来计算，最后判断大小是否超过了[−2³¹,  2³¹ − 1]这个范围。时间复杂度为O(log ₁₀ x), 空间复杂度为O(1)。

代码

 class Solution(object):

     def reverse(self, x):

         """

         :type x: int

         :rtype: int

         """

         if x == :                     # 为0直接返回

             return

         res, neg_flag = ,False         # 设置结果和负数标志位

         if x < :

             neg_flag = True

             x = abs(x)

         while x:                      # 进行反转

             tem = x %

             x = x//

             res = (res* + tem)

         if neg_flag:                   # 判断舒服标志位，并进行相应的转换

             res =  -res

         if res > pow(,)- or res < -pow(,):      # 判断是否超出范围， 直接返回0

             return

         return res                  # 返回结果