No. 03Build it, then explain it
Technical interview practice in your browser
Warm up for coding interviews without setting anything up. Pick an exercise, write JavaScript or Python in the browser, run the public tests, then sketch the architecture and explain your trade-offs as you would to an interviewer.
Free during early access — create a free account first, no card.
01Workflow
How it works
- 01
Pick an exercise
Choose a coding exercise and a language. Clarify inputs and edge cases before you write code.
- 02
Run the tests
Execute your solution against public tests in the browser and iterate until it passes.
- 03
Explain the trade-offs
Take the solution to the interviewer and defend its complexity and design choices.
02Fit
Technical lab: who it’s for and when to use it
What’s includedTechnical lab
- 01JavaScript and Python run in an in-browser sandbox, with nothing to install.
- 02Run public test cases and compare expected with actual output.
- 03A system-design whiteboard for components and the connections between them.
- 04Save drafts, then discuss your solution with the interviewer.
Who it’s for
- Engineers and students warming up for coding rounds who want quick feedback without a local setup.
- Candidates who can solve a problem but struggle to explain its complexity and edge cases out loud.
- Anyone who wants to pair a coding exercise with a quick system-design sketch.
When to use it
- Short daily warm-ups in the weeks before a technical loop.
- After your tests pass, to rehearse the follow-up question an interviewer would ask next.
- Before a system-design round, to draw components and connections and talk them through.
03Sample output
A lab run against the public tests
The exercise and its tests below are the lab’s own; the results are computed from the sample solution.
Pair sum · PythonSample
Draft · target O(n) / O(n)
def solution(nums, target):
seen = {}
for i, n in enumerate(nums):
if target - n in seen:
return [seen[target - n], i]
seen[n] = i
return []Public tests · 4/4
| Input | Expected | Result |
|---|---|---|
[2,7,11,15], 9 | [0,1] | Pass |
[3,3], 6 | [0,1] | Pass |
[-2,4,6], 4 | [0,2] | Pass |
[1,2], 9 | [] | Pass |
AI review · excerpt
- Complexity
- O(n) time, O(n) extra space — matches the target.
- Edge case to discuss
- Duplicates such as [3, 3]: the index is stored after the check, so one element is never used twice.
- Follow-up question
- If the input were sorted, how would you solve it with O(1) extra space?
Sample run on the lab’s real exercise and public tests; the code and the review excerpt are illustrative. Passing public tests doesn’t prove a solution is correct for every input.
04Limits
Technical lab: limits and honest caveats
- The exercise set is small and focused on common patterns such as hash maps, stacks, intervals, sliding windows and caches. It is a warm-up, not a full problem archive.
- Only the public tests run. Passing them doesn’t prove a solution is correct for every input.
- Each run has a short time limit (about 2 seconds for JavaScript, 3 for Python), and the first Python run takes a few seconds to load the Pyodide runtime.
- AI reviews point out bugs, complexity and edge cases with hints. They don’t hand you a finished solution.
05Questions
Technical lab: common questions.
Answers specific to this tool, based on how it actually works. General questions are on the home page.
Read the guides01Which programming languages are supported?
JavaScript and Python. Every exercise has a starter in both, and the same public tests run in either language.
02Does the lab review my code?
Yes, when you ask. The AI review lists correctness risks with fix hints, the Big-O time and space of the code you submitted, edge cases worth discussing, readability notes and one follow-up question an interviewer might ask.
03Is there a system-design whiteboard?
Yes. Add components and the connections between them, save the diagram with your draft and include it in an AI review.
06Works well with
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