Introduction to the LeetCode System

Introduction

LeetCode is an online problem-solving platform that focuses on algorithmic and data structure challenges, expressed as precise computational problems with well-defined input–output behavior. At a technical level, it can be viewed as a curated set of problem specifications paired with automated evaluation logic. Each problem defines constraints, edge cases, and expected behavior, enabling consistent and deterministic validation of solutions across multiple programming languages.

From a System Design perspective, LeetCode operates as a distributed code execution and grading platform. Its core features include the ability to submit solutions in multiple programming languages, automated evaluation against visible and hidden test cases, and consistent enforcement of correctness, time, and memory constraints. Submissions are executed in isolated environments, and results are aggregated to provide deterministic feedback on correctness and performance. In this way, LeetCode serves as a practical reference for designing scalable, automated code assessment systems, rather than simply a collection of interview-style problems.

The following illustration shows the high-level workflow of a LeetCode-style system from problem selection to evaluation:

In practice, the platform supports browsing and solving coding problems by topic and difficulty, submitting solutions for automated evaluation, participating in time-bound contests with real-time leaderboards, and engaging in discussions around problems and solutions. It also maintains user history and performance metrics and offers premium features such as mock interviews and company-specific problem sets. Together, these capabilities define the functional surface area that the underlying system must support at scale.

In this chapter, we will focus on designing a system similar to LeetCode. Let’s start by identifying the requirements for such a system.

Requirements of LeetCode’s design

Because LeetCode provides many features, its functional requirements are extensive. To keep this design problem focused, we’ll limit the scope to the following functional and non-functional requirements:

Functional requirements

We will consider the following essential functional requirements for designing a LeetCode system:

• List questions: The system should allow users to browse practice problems by ...