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Lyft·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
May 2026

Summary

Lyft software engineer interview with a file processing problem that had more edge cases than I expected. The question itself sounds simple but the follow-up constraints are where they actually test you.

Questions Asked (1)

Q1

Given a large UTF-8 text file, write a program that reads the K-th non-empty line without loading the entire file into memory. How do you handle edge cases like fewer than K non-empty lines, blank lines, and trailing newlines?

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

The core read-and-count loop took me maybe five minutes to sketch out.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem constraints and edge cases, then propose a streaming solution that reads the file line by line, counting only non-empty lines until reaching K. Discuss how to handle edge cases such as fewer than K non-empty lines, blank lines, and trailing newlines, and consider trade-offs like memory usage and time complexity.

Pro tip: Mention that you would use a buffered reader to efficiently read lines without loading the entire file, and explicitly define what constitutes a 'non-empty line' (e.g., lines with only whitespace) to avoid ambiguity.

1. Clarify requirements and edge cases

Ask questions to confirm the definition of a non-empty line (e.g., does it include whitespace-only lines?), the expected behavior when K is invalid (e.g., K <= 0), and how to handle files with fewer than K non-empty lines.

2. Design a streaming algorithm

Propose reading the file line by line using a buffered reader, maintaining a counter for non-empty lines. When the counter reaches K, return the current line; if EOF is reached before K, return an appropriate error or sentinel value.

3. Address edge cases

Explain how to handle blank lines (skip them), trailing newlines (they produce no extra line), and files with fewer than K non-empty lines (return null or throw an exception). Also consider K <= 0 as invalid input.

4. Analyze complexity and trade-offs

State that the time complexity is O(N) where N is the number of lines read until the K-th non-empty line, and space complexity is O(1) aside from the buffer. Discuss alternatives like indexing if multiple queries are expected.

5. Provide code or pseudocode

Write a concise code snippet (e.g., in Python or Java) that implements the solution, demonstrating proper resource management (e.g., using try-with-resources or context managers).

Key Points to Mention

  • Use a buffered reader to read line by line without loading the entire file into memory.
  • Define 'non-empty line' clearly: typically a line that contains at least one non-whitespace character, or simply not empty after stripping newline.
  • Handle K <= 0 as invalid input, and return null/throw exception if fewer than K non-empty lines exist.
  • Trailing newline at end of file does not create an extra empty line; most line readers handle this correctly.
  • Time complexity O(N) where N is the number of lines read until the K-th non-empty line; space complexity O(1) excluding the buffer.
  • Consider trade-offs: if multiple K values are queried, building an index of line offsets might be beneficial despite initial O(N) scan.

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