0007: Reverse Integer

Problem Statement

Given a signed 32-bit integer x, return x with its digits reversed. If reversing x causes the value to go outside the signed 32-bit integer range [-231, 231 - 1], then return 0.

Assume the environment does not allow you to store 64-bit integers (signed or unsigned).

Example 1:

Input: x = 123
Output: 321

Example 2:

Input: x = -123
Output: -321

Example 3:

Input: x = 120
Output: 21

Constraints:

  • -231 <= x <= 231 - 1

Code Solution

class Solution:
    def reverse(self, x: int) -> int:
        
        # record the sign
        sign = -1 if x < 0 else +1
        
        # set limits without overflowing the values
        limit = 2 ** 30 + ((2 ** 30) - 1) if sign == +1 else 2 ** 31
        
        x = abs(x)
        
        cnum = 0
        while x:
            
            # look ahead if overflow can happen
            if cnum > limit // 10:
                return 0
            
            # corner case for overflow comparing last digit
            if cnum == limit // 10 and (x % 10 > limit % 10):
                return 0
            
            # update the value:
            cnum = cnum * 10 + (x % 10)
            x = x // 10
        
        return sign * cnum