L1 · FoundationsList basics~2 min · 4 tests

Reverse a List In Place

Reverse a Python list in place with two pointers and swaps in O(n) time and O(1) extra space — no reverse() or slicing. Live tests check it mutates the input.

The problem

Reverse nums in place (modify the list itself) and return it. Don't use reverse(), reversed() or slicing.

Examples

Example 1

Input

reverse_in_place([1, 2, 3, 4])

Expected output

[4, 3, 2, 1]

Example 2
Input
```
reverse_in_place([1, 2, 3])
```
Expected output
```
[3, 2, 1]
```

+ 2 hidden tests on Submit — original list is changed.

Edge cases to ask about

Empty
Odd length
Even length

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_in_place

def reverse_in_place(nums):
    """Reverse nums in place and return it."""
    # Write your solution here
    pass

⌘/Ctrl + Enter runs the examples

Your code runs in real CPython inside your browser — nothing is sent anywhere. The first run downloads the interpreter (about 6 MB, once). Your code is saved on this device as you type.

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 measure your code against the optimal one at growing input sizes.

Time complexity of the optimal solution

Space complexity (extra memory)

Pick both to reveal the answer.

Measure it

Runs the function on inputs of size 250 up to 16,000 and records the time and peak memory. Slow solutions stop early — a short curve is itself the answer.

From brute force to optimal

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

Approach	Time	Space	Idea
Build a reversed copy	O(n)	O(n)	Not in place.
bestSwap from both ends	O(n)	O(1)	Tuple assignment swaps without a temp variable.

Walkthrough of the optimal approach (try it yourself first)

Two pointers at the ends, swap, move inward, stop when they meet. nums[i], nums[j] = nums[j], nums[i] swaps without a temp variable. Returning a new list (nums[::-1]) would not be in place — the hidden test checks the original list changed.

Complexity: O(n) time, O(1) space. n / 2 swaps; only two indices of extra memory.

Reveal the reference solution

def reverse_in_place(nums):
    i, j = 0, len(nums) - 1
    while i < j:
        nums[i], nums[j] = nums[j], nums[i]
        i += 1
        j -= 1
    return nums

Follow-ups interviewers ask

Reverse only the sub-range i..j.

Frequently asked interview questions

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

What is the time complexity of Reverse a List In Place in Python?

The optimal solution runs in O(n) time and O(1) auxiliary space. n / 2 swaps; only two indices of extra memory.

What is the brute-force approach, and how do you optimise it?

Build a reversed copy: O(n) time, O(n) space. Not in place. Swap from both ends: O(n) time, O(1) space. Tuple assignment swaps without a temp variable.

What follow-up questions do interviewers ask about Reverse a List In Place?

Reverse only the sub-range i..j.