Reddit Interview Questions — Page 2

Reddit·Machine Learning Engineer·Onsite - System Design / Architecture

Apr 2026

Reddit ML Engineer system design round focused entirely on building a comment ranking system from scratch. Pretty deep dive, covering everything from the scoring function to cache invalidation to an ML layer on top. Felt like three interviews crammed into one.

Design a ranking system for comments on a post, similar to Reddit or Hacker News. Walk through the full system including functional requirements, ranking logic, architecture, scale, and any ML components.

“This question is basically five questions wearing a trench coat.”

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Reddit·Software Engineer·Onsite - System Design / Architecture

May 2026

Reddit system design round, one big question about trending/hot posts. Pretty open-ended and they clearly wanted you to go deep on multiple dimensions at once, which I was not fully prepared for.

Design a system that surfaces the hottest posts over a configurable time window (e.g. last hour or last day), covering event ingestion, ranking signal aggregation, time-decay, top-K maintenance, storage layout, and real-time vs batch tradeoffs.

“I started with ingestion and got a bit stuck on the time-decay part longer than I should have.”

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Reddit·Software Engineer·Technical Phone Screen

May 2026

Reddit SWE interview that involved a log processing problem with a follow-up about scaling to millions of entries. Pretty systems-heavy for what felt like a mid-level coding round.

Given a log of forum activity, add the sub-forum ID as the first column and perform aggregation or analysis operations broken out per sub-forum. Then, how would you optimize this if the log grows to millions of entries?

“The first part was manageable, just restructuring the data and grouping by sub-forum.”

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Reddit·Machine Learning Engineer·Technical Phone Screen

May 2026

Reddit MLE interview touched on a classic but tricky sequence manipulation problem. The pressure came from needing to handle multiple variants back to back, not just one clean version of the problem.

You're given a base sequence and a stream of operations (inserts, deletes, updates, range updates). Design a solution that handles these modifications efficiently and can answer queries about the resulting sequence. Be prepared to solve at least two variants: one with point updates and range queries, another with range updates and point queries.

“The first variant felt manageable but I started second-guessing my data structure choice midway through.”

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Reddit·Data Scientist·Technical Phone Screen

May 2026

SQL round for a Data Scientist role at Reddit. Two questions, both centered on user/post data with some real edge cases baked in. Not the hardest SQL I've seen but the details trip you up if you're not careful.

Given a users table and a posts table, write a SQL query to classify each user as 'in the US' or not. A user counts as US-based if their home country is US, or if at least 50% of their posts in the last 30 days came from a US IP. Return user_id, a binary flag, and the share of US posts over the last 30 days (null if they have no recent posts).
Write a SQL query to find the top 10 most active forums over the last 30 days, ranked first by total post count and then by distinct poster count as a tiebreaker. Return forum_id, forum_name, post count, and active user count.

“This one has more moving parts than it looks.”

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Reddit·Software Engineer·Onsite - System Design / Architecture

May 2026

System design round at Reddit for a software engineer role, focused entirely on building a notification platform at scale. Pretty deep dive, they wanted the full picture from API contracts down to dead-letter queues.

Design a notification system that can deliver messages to users across push, email, SMS, and in-app channels at millions of notifications per minute.
How would you design the producer ingest API and the channel adapter interface for a multi-channel notification system?
Walk through how you'd handle deduplication, retries with exponential backoff, and dead-letter queue management in this notification pipeline.
How would you approach rate limiting per notification provider and ensuring you don't overwhelm downstream channels like SMS gateways?
How would you handle observability for this system, and what metrics or signals would you prioritize?

“This is the kind of question where you can spiral fast if you don't anchor on requirements first.”

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Reddit·Software Engineer·Technical Phone Screen

May 2026

Reddit coding screen for a software engineer role, one meaty question about merging overlapping chat context windows efficiently. The problem looked like a straightforward dedup task until the time complexity constraint showed up.

You have a chat history where messages have unique strictly-increasing integer IDs. An external API returns a context window around a given message ID (up to N messages before and after it). Given a list of IDs, implement a class that fetches the context window for each ID and returns a single deduplicated, sorted list of messages, running in better than O(M log M) time where M is the total number of fetched messages.

“The dedup part I figured out pretty fast, just track seen IDs in a set.”

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Reddit·Machine Learning Engineer·Technical Phone Screen

May 2026

Reddit ML Engineer interview with a hands-on data prep coding question. The whole thing was basically a Jupyter notebook exercise where you had to wrangle a JSON dataset into something model-ready, which felt more like a data engineering warmup than a proper ML interview.

Given a JSON dataset with fields for hours spent reading different post categories, a current post category label, and a binary click outcome, load it into a pandas DataFrame, enforce correct data types, one-hot encode the categorical field, validate the schema, and confirm there are no missing values or class imbalance issues.

“This looked straightforward and I almost got complacent.”

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Reddit·Software Engineer·Onsite - System Design / Architecture

May 2026

Reddit system design round, one big question about building a live chat system from scratch. Covered a lot of ground and I think I did okay but the group chat fan-out piece got a bit messy toward the end.

Design a live chat system that supports both 1:1 and group conversations, including message delivery, presence indicators, read receipts, message history, and offline push notifications.

“I started with requirements and that part went fine.”

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Reddit·Machine Learning Engineer·Technical Phone Screen

May 2026

Reddit ML Engineer interview that was basically one big applied ML design question. They gave you a click-through prediction scenario and expected you to walk through the whole pipeline from baseline to production-ready model, justifying every choice along the way. Dense but kind of interesting if you like that stuff.

Build a click-through rate prediction model: start from a trivial baseline, then train and compare logistic regression, random forest, and gradient-boosted trees. Justify why you chose each and why you skipped alternatives like SVMs, Naive Bayes, or simple neural nets, covering bias-variance tradeoffs, interpretability, and compute cost.
Describe your cross-validation strategy and which hyperparameters you would tune for each of the three models.
Choose appropriate evaluation metrics for a roughly class-balanced dataset and justify them. Then explain what would change if the classes were heavily imbalanced.
What additional improvements would you pursue if you had more time to iterate on this model?

“This is where I spent most of my energy.”

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