L1 · FoundationsNumbers & loops~4 min · 5 tests

Reverse the Digits of an Integer

Reverse an integer's digits arithmetically, so 1234 becomes 4321 and -120 becomes -21, without converting to a string. Python interview basics, run live.

The problem

Return n with its digits reversed, keeping the sign. Trailing zeros disappear: -120 → -21. Use arithmetic, not strings.

Examples

Example 1
Input
```
reverse_int(1234)
```
Expected output
```
4321
```
Example 2
Input
```
reverse_int(-120)
```
Expected output
```
-21
```

+ 3 hidden tests on Submit.

Edge cases to ask about

Negative numbers
Trailing zeros
Zero

Hints

0/3

How an interviewer scores this

0/9

1. Understand the problemRestate it, ask about edge cases and input size.
2. Write a brute-force solutionCorrect first, clever later.
3. Get the correct outputautoAll visible examples pass.
4. Analyse time complexityautoCommit to a Big-O in the Complexity Lab.
5. Analyse space complexityautoExtra memory, not the input.
6. Identify the bottleneckautoMeasure your code — where does it bend?
7. OptimiseUsually: a set or dict instead of a scan.
8. Explain why it worksSay it out loud to Solvi.
9. Test the edge casesautoSubmit passes every hidden test.

Python 3.13 · reverse_int

def reverse_int(n):
    """Reverse the digits of n, keeping its sign."""
    # Write your solution here
    pass

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Complexity Lab

What does this cost as n grows?

Interviewers score the analysis as much as the code. Commit to an answer first — then check it, and read why.

Time complexity of the optimal solution

Space complexity (extra memory)

Pick both to reveal the answer.

From brute force to optimal

The progression an interviewer wants to hear, one step at a time.

Approach	Time	Space	Idea
Pop and push digits	O(log n)	O(1)	rev = rev * 10 + n % 10

Walkthrough of the optimal approach (try it yourself first)

Peel the last digit off n and push it onto rev with rev = rev * 10 + digit. Work on abs(n) because Python's % with negatives returns a positive remainder (-7 % 10 == 3), then reapply the sign.

Complexity: O(log n) time, O(1) space. One iteration per digit.

Reveal the reference solution

def reverse_int(n):
    sign = -1 if n < 0 else 1
    n = abs(n)
    rev = 0
    while n:
        rev = rev * 10 + n % 10
        n //= 10
    return sign * rev

Follow-ups interviewers ask

Return 0 if the result overflows 32 bits (LeetCode 7).

Frequently asked interview questions

Core interview concepts, complexities, and follow-ups scored by hiring teams.

What is the time complexity of Reverse the Digits of an Integer in Python?

The optimal solution runs in O(log n) time and O(1) auxiliary space. One iteration per digit.

What follow-up questions do interviewers ask about Reverse the Digits of an Integer?

Return 0 if the result overflows 32 bits (LeetCode 7).