Reverse Integer

Reverse Integer with the 32 bit range check done without ever holding a 64 bit integer, which is the constraint the problem actually cares about.

January 20, 2024Updated July 30, 2026

Original: LeetCode 7. Reverse Integer

Difficulty: Medium

Topics: Math

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 [-2^31, 2^31 - 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:

-2^31 <= x <= 2^31 - 1

Solution

Reverse the digits

Approach: turn the integer into a string, reverse the string, and convert it back. The interesting half is the range check, because the problem forbids a 64 bit integer, so the comparison has to happen on the reversed value itself.

The same 32 bit boundary without a 64 bit integer comes up in "String to Integer (atoi)", where the requirement is to clamp into range rather than return 0.

Complexity: O(n)

class Solution:
    def reverse(self, x):
        res = int(str(x)[::-1]) if x >= 0 else -1 * int(str(-x)[::-1])
        return res if res < 2**31 and res >= -2**31 else 0

Result: 24ms, beats 99.42% of users with Python3.