Language Tactics Playground Academy Problems Pricing

All problemsProblems LeaderboardRanking My progressProgress

Courses →Get thePass · €10

7. Valid parentheses

MediumProgramAcademy Pass

Opening the problem…

Similar problems

16. Run-length encoding Medium
28. Longest substring without repeats Medium
1. Fizz Buzz Easy
3. Palindrome Easy

← 6. Climbing stairs 8. Gauss’s sum →

A comprehensive learning platform for the Lean 4 programming language and theorem prover.

Lean 4 on GitHub

Learn

Language Guide
Tactics Reference
Lean Playground
Interactive Academy
Lean Problems
Proof Stories
Pricing

Community

Community Forum
Lean Zulip
Official Lean Website
Mathlib Documentation

Site

About
Contact
Editorial Policy
Your Account

Policies

Privacy Policy
Terms of Use
Withdraw from contract
Legal Notice

Built for the Lean community. Not officially affiliated with the Lean FRO. Lean is a trademark of Lean FRO, LLC.