ElevenLabs·Software Engineer·Technical Phone Screen·Senior

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May 2026Remote

Summary

ElevenLabs SWE interview with a meaty systems-style coding problem about file permission inheritance. One question but it had enough layers to keep me busy for the whole session.

Questions Asked (1)

Given a hierarchical file system where folders can allow or deny access to users or groups, and permissions propagate down unless overridden, write a function to check whether a specific user has access to a file at a given point in time. Permission changes are timestamped, so historical states matter.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

The timestamp angle is what tripped me up at first.

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AI HintsAI Generated

Suggested Approach

Clarify the data model and requirements first, then design a solution that reconstructs the permission state at the given timestamp by processing only relevant changes. Use a tree traversal from the file up to the root, applying the most recent permission rule at each level, and handle overrides and group memberships efficiently.

Pro tip: Mention that you would preprocess permission changes into a timeline per node and use binary search to find the effective rule at the query time, turning a potentially O(N) scan into O(log N) per node. Also note that caching or memoizing results for repeated queries can be a practical optimization in production.

1. Clarify requirements and assumptions

Ask about the data model: how permissions are stored (per folder, per user/group), how timestamps are represented, and whether group memberships also change over time. Confirm that access is determined by the most specific rule (file-level overrides folder-level) and that deny takes precedence over allow if both apply.

2. Design the data structures

Represent the file system as a tree where each node stores a list of permission change events sorted by timestamp. Each event includes the user/group, allow/deny, and timestamp. Also maintain a mapping of users to groups (with time ranges if memberships change).

3. Reconstruct permission state at query time

For the given file and timestamp, traverse from the file node up to the root. At each node, find the most recent permission event affecting the user (directly or via groups) using binary search on the sorted events. Collect these effective rules.

4. Resolve conflicts and determine access

Apply the rules from the most specific (file) to least specific (root). The first rule that matches the user (directly or via group) determines access, unless a deny rule at a higher level overrides? Actually, typical semantics: the most specific rule wins; if none, default deny. Explain your chosen precedence and justify it.

5. Analyze complexity and trade-offs

Discuss time complexity: O(depth * log(events per node)) with binary search, or O(depth * events) if scanning. Mention space-time trade-offs, such as precomputing effective permissions per user at each timestamp (expensive) vs. on-the-fly computation. Suggest caching for repeated queries.

Key Points to Mention

Permission propagation and override semantics: child rules override parent rules, and deny typically takes precedence over allow.
Time-travel queries: need to consider only permission changes with timestamp <= query time, and group memberships at that time.
Efficient lookup: binary search on sorted permission change events per node to find the effective rule at a given timestamp.
Group membership handling: users can belong to multiple groups, and group permissions apply; memberships may also change over time.
Conflict resolution: when multiple rules apply (e.g., user-specific vs. group, allow vs. deny), define clear precedence (e.g., most specific wins, deny overrides allow).
Scalability and caching: for repeated queries, cache results or precompute effective permissions; consider trade-offs between memory and query latency.

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