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.

• 簡單題，相似字符串處理。主要是考慮integer overflow狀況。比較tricky的方法是用long來存儲result結果，而後判斷是否超過INT_MAX/INT_MIN。
• 我用的仍是判斷overflow。因爲輸入不多是INT_MAX/INT_MIN的reverse，因此不須要考慮abs(INT_MIN) > abs(INT_MAX)的狀況。

 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 <iostream>
10 #include <cstring>
11 #include <vector>
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_MIN
27                 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<int> iVec = {123, -123, 120, 2147483647, -2147483648, 0};
43     
44     /*
45      321
46      -321
47      21
48      0
49      0
50      0
51      */
52     for (auto i : iVec) {
53         cout << testSolution.reverse(i) << endl;
54     }
55 
56     return 0;
57 }

View Code

---

• 一種解法同上，字符串處理。注意Python3中整除用//，而不是/。
• 第二種解法利用Python sliding s[::-1]作字符串反轉。
• Python3 字符串 | 菜鳥教程
  • 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. 

 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 0
13             
14             result = result * 10 + x % 10
15             x //= 10 # be careful of floor division (integer division)
16         
17         return result * sign
18     
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_reverse
26         else:
27             return 0

View Code