https://leetcode.com/problems/reverse-integer/description/

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

Example 1:

Input: 123Output:  321

Example 2:

Input: -123Output: -321

Example 3:

Input: 120Output: 21

Note:

Assume we are dealing with an environment which could only hold integers within the 32-bit signed integer range. For the purpose of this problem, assume that your function returns 0 when the reversed integer overflows.

• 简单题，类似字符串处理。主要是考虑integer overflow情况。比较tricky的方法是用long来存储result结果，然后判断是否超过INT_MAX/INT_MIN。
• 我用的还是判断overflow。由于输入不可能是INT_MAX/INT_MIN的reverse，所以不需要考虑abs(INT_MIN) > abs(INT_MAX)的情况。

1 // 2 //  main.cpp 3 //  LeetCode 4 // 5 //  Created by Hao on 2017/3/16. 6 //  Copyright © 2017年 Hao. All rights reserved. 7 // 8  9 #include 
      
       10 #include 
       
        11 #include 
        
         12 using namespace std;13 14 class Solution {15 public:16     int reverse(int x) {17         int result = 0;18         int sign = 1;19         20         if (x < 0) {21             sign = -1;22             x = abs(x);23         }24         25         while (x != 0) {26             if (result > (INT_MAX - x % 10) / 10) // it's ok coz input would not be reverse of INT_MIN27                 return 0;28 29             result = result * 10 + x % 10;30             x /= 10;31         }32         33         return result * sign;34     }35 };36 37 int main(int argc, char* argv[])38 {39     Solution    testSolution;40     string      result;41     42     vector
         
           iVec = {
    123, -123, 120, 2147483647, -2147483648, 0};43     44     /*45      32146      -32147      2148      049      050      051      */52     for (auto i : iVec) {53         cout << testSolution.reverse(i) << endl;54     }55 56     return 0;57 }
         
        
       
      

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• 一种解法同上，字符串处理。注意Python3中整除用//，而不是/。
• 第二种解法利用Python sliding s[::-1]做字符串反转。
• Python3 字符串 | 菜鸟教程
  • http://www.runoob.com/python3/python3-string.html
• python - Maximum and Minimum values for ints - Stack Overflow
  • https://stackoverflow.com/questions/7604966/maximum-and-minimum-values-for-ints
  • Python 3
    • In Python 3, this question doesn't apply. The plain int type is unbounded.
    • However, you might actually be looking for the machine's . That's still available in Python 3 as sys.maxsize.
  • Python 2
    • In Python 2, the maximum value for plain int values is available as sys.maxint:
    • >>> sys.maxint 9223372036854775807
  • You can calculate the minimum value with -sys.maxint - 1 as shown .
  • Python seamlessly switches from plain to long integers once you exceed this value. So most of the time, you won't need to know it. 

1 class Solution: 2     def reverse(self, x: int) -> int: 3         result = 0 4         sign = 1 5          6         if x < 0: 7             sign = -1 8             x = abs(x) 9             10         while x != 0:11             if result > ( 2**31 - ( x % 10 ) ) // 10: # be careful of floor division (integer division)12                 return 013             14             result = result * 10 + x % 1015             x //= 10 # be careful of floor division (integer division)16         17         return result * sign18     19     def reverse2(self, x: int) -> int:    20         # [begin:end:step] - leaving begin and end off and specifying a step of -1, it reverses a string.21         s = str( abs( x ) )22         x_reverse = int( s[ ::-1 ] ) if x > 0 else (- int( s[ ::-1 ] ) )23         24         if ( ( x_reverse >= - 2**31 ) and ( x_reverse <= 2**31 - 1 ) ):25             return x_reverse26         else:27             return 0