Reverse Integer

Math

15 min

Problem Statement

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

Example

Example 1:

Input: x = 123

123321

Output: 321

Solution (Pop and Push Digits)

To reverse an integer, we can repeatedly "pop" the last digit off the number and "push" it onto the back of a new reversed number. We must also be careful about potential integer overflow.

Algorithm Steps

Initialize a reversed variable to 0.
Loop while the input number x is not 0.
In each iteration, get the last digit of x (the "pop" operation).
Before adding the new digit to reversed, check if doing so would cause an overflow. If it would, return 0.
"Push" the popped digit onto reversed by multiplying it by 10 and adding the digit.
Remove the last digit from x.
After the loop, return the reversed number.

Input (x):

Remaining (x)

123

Reversed (rev)

Start with x = 123.

Reverse Integer Solution


class Solution:
    def reverse(self, x: int) -> int:
        sign = -1 if x < 0 else 1
        x *= sign
        
        rev = 0
        while x > 0:
            rev = rev * 10 + x % 10
            x //= 10
            
        if rev > 2**31 - 1:
            return 0
            
        return rev * sign