AI Interview Simulator

Practice coding interviews before they matter

AI-generated problems, live code execution, and real-time interviewer feedback that feels closer to the real thing than a static problem bank ever will.

See How It Works

No account required · Works in your browser · Built with OpenAI Codex

nullcase.dev/interview/abc123
Two SumEasy

Given an array of integers nums and an integer target, return the indices of the two numbers that add up to the target. Solve it in one pass if you can.

Example input
nums = [2, 7, 11, 15], target = 9
Expected output
[0, 1]
Constraints: 2 ≤ nums.length ≤ 10⁴
Follow-up: can you do better than O(n²)?
python · running hidden tests
42:18 elapsed
def two_sum(nums, target):
seen = {}
for index, value in enumerate(nums):
complement = target - value
if complement in seen:
return [seen[complement], index]
seen[value] = index
return []
✓ 3/3 test cases passed
∞

Infinite unique problems

10s

10 seconds to start

4

4 languages

$0

Always free

Unique Problems

Fresh prompts that don't feel pulled from a memorized bank.

Every interview starts with a newly generated problem tuned to your chosen topic and difficulty, so practice stays uncomfortable in the right way.

Merge IntervalsMedium

Given an array of intervals where intervals[i] = [start, end], merge all overlapping intervals and return an array of the non-overlapping intervals that cover all the ranges in the input.

Input
[[1,3],[2,6],[8,10],[15,18]]
Output
[[1,6],[8,10],[15,18]]
Category: sorting
Target complexity: O(n log n)
1def longest_substring(s):
2 left = 0
3 best = 0
4 last_seen = {}
5
6 for right, char in enumerate(s):
7 if char in last_seen and last_seen[char] >= left:
8 left = last_seen[char] + 1
9
10 last_seen[char] = right
11 window = right - left + 1
12 if window > best:
13 best = window
14
15 return best

Live Execution

Run your code in real time.

Write full solutions, not toy snippets. Hit run and see whether your logic survives actual execution before the interviewer moves on.

✓ 3/3 test cases passed

Feedback As You Go

It pushes back while you're still thinking.

I passed the examples, but I'm still checking duplicate values.
Good progress. Your approach works, but walk me through the time complexity if the input grows to 10,000 elements.
can I optimize with a hash map?
Yes. That tradeoff can get you to linear time. What extra space would you be paying for, and when is that worth it?

Get Started

Up and running in seconds.

No downloads, no setup ceremony, no waiting around for the product to prove it can do something useful.

01

Choose your setup

Pick the difficulty, topic, and language and drop straight into the interview.

02

Solve under pressure

Write code, run it, and respond while the interviewer pushes on your reasoning.

03

Review the debrief

See how your correctness, complexity, and communication held up after the session.

Why Nullcase

Built for practice that actually feels like pressure.

Other tools

✗ Static problem banks you can memorize

✗ No feedback until you're done

✗ Never know if your code actually ran

✗ Generic hints that give away the answer

Nullcase

✓ Unique problem every session

✓ Real-time interviewer feedback

✓ Code executed against hidden test cases

✓ Pushback without spoiling the solution

Start Practicing

Ready to try Nullcase?

Pick a difficulty, choose a topic, and step into an interview that actually asks something new.

Contest deadline: April 30, 2026