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Dropbox·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
Jul 2026Remote

Summary

Dropbox SWE interview with a file system access problem. Pretty well-scoped question but the follow-up about efficiency tripped me up more than I expected.

Questions Asked (1)

Q1

Design and implement a HasAccess function for a hierarchical file system where access to a folder is inherited by all its descendants. Given a set of directly accessible folders, return whether a user can access a given folder path.

Algorithms & Data StructuresSystem Design
Author's notes

My first instinct was to just check if the path is in the set, which is obviously incomplete.

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AI HintsAI Generated

Suggested Approach

Clarify the input format and constraints, then propose an efficient solution using a trie or hash set with ancestor checking. Discuss trade-offs between preprocessing and query time, and handle edge cases like root and trailing slashes.

Pro tip: Mention that you can preprocess the accessible folders into a trie for O(1) lookup per path segment, but also consider a simpler hash set approach if the number of accessible folders is small. Always discuss time and space complexity.

1. Clarify requirements and constraints

Ask about the input format (e.g., list of strings, tree structure), expected query frequency, and whether paths are absolute. Confirm if access is inherited strictly by prefix (e.g., '/a' grants access to '/a/b').

2. Choose data structures

Decide between a trie (prefix tree) for efficient prefix matching or a hash set of accessible folders with ancestor checks. Consider preprocessing time vs. query time trade-offs.

3. Design the algorithm

For a trie: insert all accessible folders, then traverse the query path; if any node is marked accessible, return true. For hash set: check if the query path or any of its ancestors is in the set.

4. Handle edge cases

Consider root path '/', trailing slashes, case sensitivity, and empty inputs. Ensure the algorithm correctly handles paths that are prefixes of each other.

5. Analyze complexity and optimize

State time and space complexity for both preprocessing and query. Discuss possible optimizations like caching frequent queries or using a sorted list with binary search.

Key Points to Mention

  • Inheritance means if a folder is accessible, all its descendants are accessible.
  • Use a trie (prefix tree) to efficiently check if any ancestor is accessible.
  • Alternative: store accessible folders in a hash set and check all ancestors of the query path.
  • Time complexity: O(L) per query where L is path length, with O(N*L) preprocessing for trie.
  • Space complexity: O(N*L) for trie, O(N) for hash set.
  • Edge cases: root access, trailing slashes, and paths that are prefixes of each other.

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