Reddit·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

May 2026Remote

Summary

Reddit SWE interview with a moderator privilege tracking problem that gets progressively nastier across three parts. Started manageable, ended with me questioning my life choices around linked list ordering.

Questions Asked (3)

Design a ModSystem class that initializes from a chronological log of moderation actions (each entry specifying a target user, an action of 'added' or 'removed', the acting user, and a timestamp). Implement a canRemoveMod method that checks whether one moderator has the rank to remove another, and a getModRanking method that returns all current moderators ordered from highest to lowest rank, where rank is determined by how early a user most recently became a moderator.

Algorithms & Data StructuresSystem Design

Author's notes

The ranking rule is the part that trips you up.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and edge cases, then design data structures to track current moderators and their most recent 'added' timestamps. Implement the ModSystem class with a log processor that updates state, and methods canRemoveMod and getModRanking using efficient lookups and sorting.

Pro tip: Discuss trade-offs between different data structures (e.g., hash map vs. balanced tree) and mention how to handle out-of-order logs or concurrent modifications, showing awareness of real-world scenarios.

1. Clarify Requirements and Edge Cases

Ask about log ordering, duplicate actions, invalid removals, and whether timestamps are unique. Confirm expected time complexity for methods.

2. Design Data Structures

Use a hash map to store each moderator's most recent 'added' timestamp and a set for current moderators. Consider a balanced BST or sorted list for ranking if frequent queries are needed.

3. Implement Initialization

Process the log chronologically: for 'added', update the moderator's timestamp and add to the set; for 'removed', remove from the set. Ignore invalid removals.

4. Implement canRemoveMod

Check if both users are current moderators. Compare their ranks: the one with the earlier 'added' timestamp has higher rank and can remove the other.

5. Implement getModRanking

Return all current moderators sorted by their most recent 'added' timestamp ascending (earliest first). If timestamps are equal, define a tie-breaker (e.g., user ID).

Key Points to Mention

Use a hash map to store the most recent 'added' timestamp for each moderator, enabling O(1) rank comparison.
Maintain a set of current moderators to quickly check if a user is a moderator.
For getModRanking, sort the current moderators by timestamp; if frequent calls, consider maintaining a sorted structure like a balanced BST or skip list.
Handle edge cases: removal of non-moderators, duplicate additions, and logs not in chronological order (if allowed).
Discuss time and space complexity: initialization O(n log n) if sorting, canRemoveMod O(1), getModRanking O(m log m) where m is number of current moderators.
Mention potential concurrency issues if the system is accessed by multiple threads and how to address them (e.g., locks or immutable snapshots).

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.

Extend the system to support multiple communities, where each log entry now includes a leading community field. All queries (canRemoveMod and getModRanking) become scoped to a specific community, and a user can hold moderator status independently across different communities.

System DesignData Modeling

Author's notes

Mostly a refactor, wrapping everything in a map keyed by community name.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the new data model: each log entry now has a community field, and moderator status is scoped per community. Then explain how to adapt the existing queries (canRemoveMod and getModRanking) to filter by community, and discuss the necessary changes to data storage, indexing, and API design to support multi-community moderation efficiently.

Pro tip: Emphasize that moderator status is independent per community, so a user can be a moderator in one community but not another; this means you need to store moderator assignments with a composite key (user, community) and ensure queries always include the community context.

1. Clarify requirements and assumptions

Confirm that each log entry includes a community field, and that moderator status is per-community. Ask about scale (number of communities, users, log volume) and consistency requirements.

2. Design the data model

Propose a schema where moderator assignments are stored with a composite key (user_id, community_id). Log entries include community_id. Consider using a relational table or a NoSQL document with community as a partition key.

3. Adapt queries for community scoping

Modify canRemoveMod to check if the user is a moderator in the specific community. Modify getModRanking to compute rankings within a community, aggregating actions only from that community's logs.

4. Address indexing and performance

Ensure indexes on (community_id, user_id) for moderator lookups and on (community_id, timestamp) for log queries. Discuss sharding or partitioning by community to scale.

5. Discuss API and consistency implications

Update API endpoints to include community_id as a required parameter. Consider caching moderator status per community and handling eventual consistency if using distributed storage.

Key Points to Mention

Composite key (user_id, community_id) for moderator assignments to ensure independence across communities.
Adding community_id to log entries and using it as a filter in all queries.
Indexing strategies: composite indexes on (community_id, user_id) and (community_id, timestamp).
Partitioning or sharding by community_id to distribute load and improve scalability.
API changes: all moderation-related endpoints must accept community_id.
Caching moderator status per community to reduce database hits, with invalidation on changes.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.

Add a demote operation to the multi-community system. Calling demote on a user should move them exactly one position lower in that community's current moderator ranking. If the user is already last or isn't a moderator, nothing happens. Subsequent calls to getModRanking and canRemoveMod must reflect the updated order.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

This is where things got messy for me.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the data structures used to store moderator rankings and how getModRanking and canRemoveMod are implemented. Then, design the demote operation to swap the user with the next lower-ranked moderator, ensuring O(1) or O(log n) time complexity and updating any auxiliary structures. Finally, discuss edge cases and how the change propagates to dependent methods.

Pro tip: Emphasize the importance of maintaining consistency across all related operations and consider thread-safety if the system is concurrent. Mention that you would add unit tests to verify the ranking updates and edge cases.

1. Understand the current system

Ask clarifying questions about the data structures (e.g., array, linked list, tree) used for moderator rankings and how getModRanking and canRemoveMod are implemented. Identify any constraints like time complexity or concurrency.

2. Design the demote operation

Propose an algorithm to find the user's current position and swap them with the next lower-ranked moderator. If the user is last or not a moderator, do nothing. Ensure the operation updates the underlying data structure efficiently.

3. Update dependent methods

Explain how getModRanking and canRemoveMod will reflect the new order. If these methods rely on cached data or indices, describe how to update them. Consider if any other methods are affected.

4. Handle edge cases and complexity

Discuss edge cases: user not a moderator, user already last, empty community, single moderator. Analyze time and space complexity of the demote operation and compare with alternatives.

5. Test and validate

Outline a testing strategy: unit tests for demote, integration tests with getModRanking and canRemoveMod, and concurrency tests if applicable. Mention potential pitfalls like stale references.

Key Points to Mention

Choice of data structure (e.g., doubly linked list for O(1) swaps, or array with index mapping)
Time complexity of demote and how it affects overall system performance
Maintaining consistency between moderator ranking and permissions (canRemoveMod)
Handling edge cases: user not a moderator, already last, empty community
Thread-safety and concurrency considerations if the system is multi-threaded
Testing strategy to ensure correctness and prevent regressions

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.