LeetCode 7. Reverse Integer

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

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 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。

//
//  main.cpp
//  LeetCode
//
//  Created by Hao on 2017/3/16.
//  Copyright © 2017年 Hao. All rights reserved.
//

#include <iostream>
#include <cstring>
#include <vector>
using namespace std;

class Solution {
public:
    int reverse(int x) {
        int result = 0;
        int sign = 1;
        
        if (x < 0) {
            sign = -1;
            x = abs(x);
        }
        
        while (x != 0) {
            if (result > (INT_MAX - x % 10) / 10) // it's ok coz input would not be reverse of INT_MIN
                return 0;

            result = result * 10 + x % 10;
            x /= 10;
        }
        
        return result * sign;
    }
};

int main(int argc, char* argv[])
{
    Solution    testSolution;
    string      result;
    
    vector<int> iVec = {123, -123, 120, 2147483647, -2147483648, 0};
    
    /*
     321
     -321
     21
     0
     0
     0
     */
    for (auto i : iVec) {
        cout << testSolution.reverse(i) << endl;
    }

    return 0;
}

---

• 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 word size. 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 here.
  • 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. 

class Solution:
    def reverse(self, x: int) -> int:
        result = 0
        sign = 1
        
        if x < 0:
            sign = -1
            x = abs(x)
            
        while x != 0:
            if result > ( 2**31 - ( x % 10 ) ) // 10: # be careful of floor division (integer division)
                return 0
            
            result = result * 10 + x % 10
            x //= 10 # be careful of floor division (integer division)
        
        return result * sign
    
    def reverse2(self, x: int) -> int:    
        # [begin:end:step] - leaving begin and end off and specifying a step of -1, it reverses a string.
        s = str( abs( x ) )
        x_reverse = int( s[ ::-1 ] ) if x > 0 else (- int( s[ ::-1 ] ) )
        
        if ( ( x_reverse >= - 2**31 ) and ( x_reverse <= 2**31 - 1 ) ):
            return x_reverse
        else:
            return 0