Coding interview challenges — practice in the browser
How coding interview practice works
LeetCode-style problems with a code editor, hints, an editorial, reference solutions and an AI code review. Work through each one the way you would in an interview: read the problem, plan an approach, write the code, then compare it with the reference and get feedback on your version.
Pick a problem
Each problem shows its difficulty, topic, a worked example, the constraints and the target time and space complexity.
Write your solution
The editor offers 16 languages and saves your code in the browser as you type.
Take a hint if you're stuck
Hints open one at a time, so you only see as much as you need. The Editorial tab explains the key idea and the approach.
Get an AI review
AI Review reads your code and returns a verdict, its time and space complexity, strengths, issues and one concrete suggestion. It needs a free account.
Compare with the reference
Every problem has a reference solution in Python, and some also in JavaScript, Java or TypeScript. Signed in, you can generate one in another language.
Example problem: Two Sum
Easy · Arrays / Hashing · Free
Given a list nums and an integer target, return the indices of the two numbers that add up to target. Exactly one solution exists.
nums = [2, 7, 11, 15], target = 9 → [0, 1] # nums[0] + nums[1] = 2 + 7 = 9
The editorial walks through the hash-map approach: one pass, O(n) time.
More problems
- Valid Palindrome — Easy · Two Pointers · free
- Maximum Subarray — Medium · Dynamic Programming · free
- Valid Parentheses — Easy · Stack
- Merge Two Sorted Lists — Medium · Two Pointers
- Time Limited Cache — Medium · Design
- Median Tracker — Hard · Heap
Frequently asked questions
Is coding practice free?
Yes, for 5 problems, with no account needed: Two Sum, Valid Palindrome, Maximum Subarray, Fizz Buzz and Reverse String. Pro ($15/mo) opens the rest. The SQL problems are free with an account.
Can I run my code?
SQL queries run against a real SQLite database (free account), and React and Next.js problems render a live preview as you type. Running other languages against test cases is switched off at the moment, so check your work with AI Review and the reference solution.
Which languages can I write in?
The editor offers 16 languages, including Python, JavaScript, TypeScript, Java, C++, C#, Go and Rust. Reference solutions are always available in Python.
Do problems come with explanations?
Yes. Each has hints, an editorial with the key idea and approach, and a reference solution with its time and space complexity.
Is there more than algorithm problems?
Yes. There are tracks for SQL, test automation (Selenium, Cypress and Playwright) and shell scripting, each with hints and reference solutions.