Design LeetCode
Last Updated: December 30, 2025
What is LeetCode?
LeetCode is an online platform where programmers practice coding problems, participate in contests, and prepare for technical interviews.
The core idea is straightforward: users browse coding problems, write solutions in their preferred programming language, submit them for evaluation, and receive immediate feedback on correctness and performance.
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The system must execute untrusted user code safely, compare outputs against test cases, and rank solutions by efficiency.
Popular Examples: LeetCode, HackerRank, Codeforces, CodeChef
This system design problem covers several interesting challenges: secure code execution, real-time judging, contest management, and leaderboard systems.
In this chapter, we will explore the high-level design of an online coding platform like LeetCode.
Let's start by understanding what exactly we need to build.