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Perplexity·Software Engineer·Technical Phone Screen·Intermediate

IntermediateRejected
Apr 2026Remote

Summary

Coding round at Perplexity for a Software Engineer role. The question was a meaty in-memory file system implementation and I got most of the way through it before running into a bug I couldn't squash in time. The interviewer was pretty checked out the whole session and I got a rejection the next day.

Questions Asked (1)

Q1

Implement an in-memory file system that supports creating directories and files, reading and writing content, and listing or searching within the file system.

System DesignAlgorithms & Data Structures
Author's notes

I'd seen this problem floating around before so I wasn't totally blindsided.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then design a tree-based data structure with nodes representing files and directories. Implement core operations (create, read, write, list, search) with efficient algorithms, and discuss trade-offs and potential optimizations.

Pro tip: Demonstrate awareness of real-world file system challenges like path normalization, concurrent access, and memory management, and mention how you would test edge cases such as circular references or invalid paths.

1. Clarify Requirements and Constraints

Ask questions to understand expected operations, scale, persistence needs, and any specific behaviors (e.g., case sensitivity, path formats). This ensures you build the right abstraction.

2. Design Data Structures

Propose a tree structure where each node represents a file or directory, storing metadata like name, type, content, and children. Consider using a hash map for quick child lookup.

3. Implement Core Operations

Write methods for creating directories/files, reading/writing content, listing directory contents, and searching (e.g., by name or content). Handle path resolution and error cases.

4. Analyze Complexity and Optimize

Discuss time and space complexity of each operation, and suggest optimizations like caching, indexing, or lazy loading for large file systems.

5. Test and Validate

Walk through test cases including edge cases (empty paths, duplicate names, deep nesting) and explain how you would verify correctness and performance.

Key Points to Mention

  • Tree-based structure with nodes for files and directories, using hash maps for efficient child lookup.
  • Path resolution and normalization (e.g., handling absolute vs relative paths, '.' and '..').
  • Time complexity: O(1) for direct child access, O(d) for path traversal where d is depth.
  • Search algorithms: BFS/DFS for listing or searching, with potential indexing for faster queries.
  • Concurrency considerations: locking mechanisms for thread safety if multiple users access simultaneously.
  • Memory management: strategies for handling large files, such as storing content in blocks or using external storage.

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