L2 · Working engineerLists & arrays~8 min · 5 tests#12

Merge Two Sorted Lists

Merge two sorted Python lists into one sorted list with two pointers in O(n+m) — no sort(). Test duplicates and empty inputs in your browser.

The problem

list1 and list2 are each sorted ascending. Return one sorted list containing every element of both.

Do it without calling sorted() or .sort().

Examples

Example 1

Input

merge_sorted([1, 3, 5, 7], [2, 4, 6, 8])

Expected output

[1, 2, 3, 4, 5, 6, 7, 8]

Example 2

Input

merge_sorted([1, 2, 2], [2, 3])

Expected output

[1, 2, 2, 2, 3]

+ 3 hidden tests on Submit — one list entirely smaller.

Edge cases to ask about

One list empty
Duplicates across lists
One list entirely before the other

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 · merge_sorted

def merge_sorted(list1, list2):
    """Merge two ascending lists into one ascending list."""
    # 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
Concatenate and sort	O((n+m) log(n+m))	O(n+m)	Ignores the fact that both inputs are already sorted.
bestTwo pointers	O(n + m)	O(n + m)	Always take the smaller head; append the leftovers at the end.

Walkthrough of the optimal approach (try it yourself first)

Hold an index i into list1 and j into list2. Repeatedly append the smaller head and advance that index. When one side is exhausted, extend with the other's remainder.

This is the merge step of merge sort, and the same idea as heapq.merge — mention that for the follow-up about k lists.

Complexity: O(n + m) time, O(n + m) space. Each element is appended exactly once; the output holds all n + m elements.

Reveal the reference solution

def merge_sorted(list1, list2):
    i = j = 0
    out = []
    while i < len(list1) and j < len(list2):
        if list1[i] <= list2[j]:
            out.append(list1[i]); i += 1
        else:
            out.append(list2[j]); j += 1
    out.extend(list1[i:])
    out.extend(list2[j:])
    return out

Follow-ups interviewers ask

Merge k sorted lists (heap, O(N log k)).
Merge in place into list1 with spare capacity at its end.

Frequently asked interview questions

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

What is the time complexity of Merge Two Sorted Lists in Python?

The optimal solution runs in O(n + m) time and O(n + m) auxiliary space. Each element is appended exactly once; the output holds all n + m elements.

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

Concatenate and sort: O((n+m) log(n+m)) time, O(n+m) space. Ignores the fact that both inputs are already sorted. Two pointers: O(n + m) time, O(n + m) space. Always take the smaller head; append the leftovers at the end.

What follow-up questions do interviewers ask about Merge Two Sorted Lists?

Merge k sorted lists (heap, O(N log k)). Merge in place into list1 with spare capacity at its end.