Design
Online Code Judge (LeetCode)
Design an online code execution and judging platform like LeetCode — users submit code, it runs in a sandbox, and results are evaluated.
This is a Low-Level Design (LLD) problem. New to LLD vs HLD? Start here.
01 - Why interviewers ask this
It combines sandboxed execution concerns with a submission-grading state machine — interviewers use it to probe whether you think about untrusted-code safety, not just the happy path.
02 - Where this system exists in the real world
You interact with this design every day.
LeetCode / HackerRank-style judge platforms
Automated grading systems for coding bootcamps
CI pipelines that run untrusted test code
03 - What you'll master
Solve this once. Know it forever.
Model a submission lifecycle and verdict states
Reason about sandboxed, resource-limited execution
Design test-case comparison and scoring
04 - What you'll design
7 functional · 4 non-functional requirements.
Problems with description, input/output format, examples, and constraints
6 more requirements inside
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05 - Companies that ask this
You may face this exact question in your next interview.
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