Reddit SWE Interview: Coding Screen Guide
Updated:
Estimated read time: 8-10 minutes
Summary: Reddit software engineer (SWE) coding screens are usually best approached as 45-60 minute engineer-led interviews. Prepare for data structures, algorithms, graph or traversal tasks, backend data manipulation, edge cases, and clear reasoning, then confirm the exact setup with your recruiter.
See the full Reddit Software Engineering interview roadmap, including each stage and how to prepare from application review to recruiter follow-up. View the Reddit Software Engineering interview roadmap
TL;DR + FAQ
At-a-glance takeaways
- Plan for a 45-60 minute live technical interview with an engineer.
- Prepare for data structures and algorithms, graph or traversal, backend data manipulation, complexity, and edge cases.
- Role variance matters: mobile, machine learning (ML), ads, safety, infrastructure, ranking, and backend roles may add different follow-ups.
- Communication is part of the signal, not a bonus.
- Senior candidates may see systems-oriented follow-up even in coding-heavy interviews.
Quick FAQ
Is this only algorithms?
Not necessarily. Backend data manipulation and role-specific follow-ups are plausible.
How should I practice?
Prepare for themes, follow-ups, and role-specific constraints.
Will mobile or ML roles differ?
They may. Confirm with the recruiter.
What matters most?
Correct code, clear reasoning, edge cases, complexity, and adaptation to follow-ups.
1) How the coding screen works
The screen is typically an engineer-led coding interview in a shared environment. Plan for 45-60 minutes, but confirm the exact tool and setup with your recruiter.
Prepare to explain your approach before coding, write a clean implementation, test edge cases, and discuss complexity. If the interviewer shifts the problem toward backend, mobile, ranking, safety, or ML context, pause and restate the new constraints before changing code.
2) Coding tasks you may face
Use these tasks to practice Reddit-relevant coding patterns and follow-ups.
- Given comments with parent IDs, reconstruct the comment tree and return the visible traversal order. Now handle missing parents and deleted comments.
- Given a stream of votes on posts, compute the current score per post. Then update the design for duplicate votes or vote changes.
- Find connected communities in a graph of users or subreddits. Now return the largest component and explain the runtime.
- Implement a rate limiter for posting or commenting. Start with one user, then support many users and sliding windows.
- Given moderation actions, determine whether a post should be visible, hidden, or pending review based on rule precedence.
- Merge two ranked lists of posts into one feed while preserving order, removing duplicates, and applying a diversity rule.
- For a mobile path, design the data transformation needed to render a feed from a paginated application programming interface (API) response.
- After the base solution works, change one constraint: larger input, out-of-order events, deleted content, or stricter latency.
Coding interviews reward visible reasoning. A mock interview helps you practice clarifying, coding, testing, and adapting when the problem changes.
3) What strong performance looks like
Strong candidates translate Reddit-flavored objects into clean data structures: posts, comments, users, votes, feeds, actions, or events. Then they name the invariant that makes the solution correct.
Testing matters. For comment trees, test missing parents and deleted nodes. For ranking, test duplicates and ties. For streams, test out-of-order updates. For rate limits, test boundary timestamps.
4) Common failure modes
Solving only the happy path. User-generated content creates deleted, duplicated, missing, and out-of-order data.
Not explaining complexity. Be ready to discuss runtime and space tradeoffs clearly.
Ignoring role context. Mobile, ML, ads, safety, infrastructure, and backend roles may use different follow-ups.
Jumping into code too fast. Clarify input, output, and invalid cases first.
No manual test. Walk through at least one normal case and one edge case.
5) How to prepare
- Review arrays, maps, sets, graphs, trees, heaps, sorting, caching, and streaming updates.
- Practice content and event problems: posts, comments, votes, feeds, moderation actions, and user events.
- For mobile paths, practice transforming API responses into user interface (UI)-ready state.
- For ML, ads, ranking, or safety paths, prepare role-specific follow-ups around scoring, data quality, and abuse cases.
- Test edge cases aloud.
Use a mock interview to rehearse coding under realistic follow-ups: deleted content, duplicate events, ranking changes, and large inputs.
See the full Reddit Software Engineering interview roadmap, including each stage and how to prepare from application review to recruiter follow-up. View the Reddit Software Engineering interview roadmap