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Liftoff·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Jun 2026

Summary

Liftoff software engineer screen focused entirely on a run-length encoding problem. Pretty classic coding exercise but they pushed harder than I expected on edge cases and complexity analysis.

Questions Asked (1)

Q1

Implement encode and decode functions for run-length encoding. encode should compress repeated characters into count-character pairs, decode should reverse it. Also discuss edge cases like empty strings and digit characters in the input, plus time and space complexity.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

I went with '1c' for singles instead of just 'c' and said so upfront, which I think was the right call since it simplifies decode a lot.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the encoding format and constraints, then implement encode using a single pass with a counter, and decode by parsing counts and characters. Discuss edge cases like empty strings, single characters, and digits in the input, and analyze time and space complexity.

Pro tip: Mention that run-length encoding is only beneficial for inputs with many consecutive repeats; for general strings, it can expand the data. Also, consider using a delimiter or fixed-width counts to handle digits in the input unambiguously.

1. Clarify requirements and constraints

Ask about the expected format (e.g., 'a3b2' vs '3a2b'), whether counts can be multi-digit, and if the input can contain digits. Confirm if the encoding should be case-sensitive and if in-place modification is required.

2. Design the encoding algorithm

Iterate through the string, count consecutive identical characters, and append the character followed by its count to the result. Handle the last group after the loop.

3. Design the decoding algorithm

Parse the encoded string by reading a character, then reading the following digits to form the count, and append the character count times to the result. Ensure multi-digit counts are handled correctly.

4. Address edge cases

Discuss empty strings, single-character strings, strings with no repeats, and inputs containing digits. For digits, propose solutions like using a delimiter or fixed-width counts, or note that the problem may assume no digits.

5. Analyze complexity and trade-offs

State that both encode and decode run in O(n) time and O(n) space for the output. Mention that encoding can increase size for non-repetitive inputs, and discuss alternative formats like using a delimiter.

Key Points to Mention

  • Time complexity: O(n) for both encode and decode, where n is the length of the input string.
  • Space complexity: O(n) for the output string; encoding may use extra space for counts.
  • Edge cases: empty string returns empty string; single character returns 'char1'; no repeats results in 'char1char1...'.
  • Handling digits in input: use a delimiter (e.g., 'a:3') or fixed-width counts (e.g., 'a03') to avoid ambiguity.
  • Trade-off: RLE is effective for data with many consecutive repeats but can expand data otherwise.
  • Decoding must correctly parse multi-digit counts (e.g., 'a12' means 12 'a's).

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