Dropbox·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

May 2026Remote

Summary

Dropbox SWE interview with a filesystem traversal problem. Pretty focused on the implementation details and the tradeoffs around recursion vs iteration, which I wasn't fully prepared to articulate under pressure.

Questions Asked (1)

Implement a function that takes a starting path and returns all files under it recursively, using only a provided `listChildren(path)` helper rather than any direct filesystem APIs.

Algorithms & Data StructuresTechnical Trade-offsSystem Design

Author's notes

I jumped straight into a recursive solution and it felt clean until they asked what happens on a really deep directory tree.

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AI HintsAI Generated

Suggested Approach

Clarify the contract of listChildren (return type, ordering, error handling) and then implement a recursive depth-first traversal that accumulates file paths. Discuss iterative alternatives and trade-offs like stack depth, memory, and handling of symlinks or cycles.

Pro tip: Mention that you would avoid recursion depth limits by using an explicit stack or queue, and that you'd consider concurrency for I/O-bound traversal, but only after confirming the helper's thread-safety.

1. Clarify requirements and assumptions

Ask about the return format (list of paths vs. tree), whether listChildren returns files and directories, and how to handle errors, symlinks, or cycles.

2. Choose traversal strategy

Decide between recursive DFS (simpler) and iterative BFS/DFS (avoids stack overflow). Explain the trade-offs in terms of memory and code clarity.

3. Implement the traversal

Write pseudocode or code: for each child, if it's a file, add to results; if it's a directory, recurse or push onto stack. Use a set to detect cycles if needed.

4. Analyze complexity and edge cases

Discuss time complexity O(N) where N is number of entries, space complexity O(D) for depth or O(N) for BFS. Mention edge cases like empty directories, permission errors, and very deep trees.

5. Optimize and extend

Suggest improvements: parallel traversal for I/O-bound scenarios, lazy iteration with generators, or early termination if searching for a specific file.

Key Points to Mention

Recursive vs. iterative traversal and stack overflow risk
Time and space complexity analysis
Handling symlinks, cycles, and permission errors
Using a queue for BFS or stack for DFS
Concurrency considerations for I/O-bound traversal
Lazy evaluation with generators to reduce memory

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