Problem 7. Reverse Integer

Difficulty: Medium
Topics: Math
LeetCode Link: Problem URL

Problem Description

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

Approach

Mathematical

Reverse the integer mathematically by extracting the last digit of it and storing it at the starting of the reversed one. Also, checking for the constraints in each step so that the reversed digit does not overflow the integer bound.

Steps:

Create a another variable temp to store the initial value of x. Make x = 0.
In a loop while temp is not equal to 0, store the digits one by one starting from the end to the starting of x.
Before actually adding the digits first check if x comes under the constraints or not. If not, return 0.
At the last return x (reversed).

String Conversion

We can also achieve reversing the digits by converting the integer to a string. For this, I chose Python for the ease of converting the data-types.

Steps:

First, create variables for the constraints as python doesn't have the INT_MAX or INT_MIN in-built.
Then, check if x is negative, if it is then convert the integer to a string with the negate of the integer s = str(-x).
Reverse the number (string) by using slicing s[::-1]
Add the negative sign back rev = -int(s[::-1])
Check if rev falls in the constraints. If not, return 0.

If x is positive then just convert it to string and reverse it and return it.

Solution

Language: C++
File: solution.cpp

Language: Python
File: solution.py

Notes

The mathemical approach is better and efficient than the string conversion one although both the approaches uses the same complexities.

It is just the execution speed which is dependant on the language we are using. C++ compiles faster than Python and has a low memory usage.

Complexity Analysis

Time Complexity: O(log x)
Space Complexity: O(1)

Test Cases

See the /tests folder for the test cases for both the languages.

Running Tests

C++ (Simple): cd tests; g++ test.cpp ../solution.cpp -o test; ./test
Python: cd tests; python test.py
Java: javac .\solution.java tests/test.java; java -cp ".;tests" test

Solved on: 26/05/2025 | Time taken: 00:15:06 | Attempts: 5