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Amplitude·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Jun 2026

Summary

Amplitude SWE interview with a compression-focused coding problem. The question had more design surface area than I expected for what seemed like a straightforward implementation task.

Questions Asked (1)

Q1

Implement a dictionary encoding compression scheme with two methods: one that encodes a list of strings into a compact format (dict part plus index sequence), and one that decodes it back to the original list. You also need to justify your delimiter choices and explain how you'd handle edge cases where words contain those delimiters.

Algorithms & Data StructuresTechnical Trade-offsSystem Design
Author's notes

I jumped straight into coding and picked comma and colon as delimiters without thinking it through.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then design the encoding format with explicit delimiters and escaping, and finally implement encode/decode with thorough edge-case handling. Justify delimiter choices based on collision probability and simplicity, and explain how escaping or length-prefixing resolves delimiter conflicts.

Pro tip: Mention that you would use a delimiter that is unlikely to appear in the input (e.g., a non-printable character like '\x1F') and implement an escaping mechanism (e.g., backslash escaping) to handle cases where the delimiter does appear. This shows foresight and robustness.

1. Clarify requirements and constraints

Ask about input size, character set, performance needs, and whether the encoded format must be human-readable or space-efficient. This guides delimiter and escaping choices.

2. Design the encoding format

Choose a delimiter (e.g., '\x1F') and an escaping strategy (e.g., backslash escape) to handle words containing the delimiter. Define the dictionary part as unique words joined by the delimiter, and the index sequence as integers joined by the same delimiter.

3. Implement encode

Build a dictionary of unique words, map each word to its index, and produce the encoded string: dictionary words (escaped) joined by delimiter, then a separator (e.g., '\x1E'), then indices joined by delimiter.

4. Implement decode

Split the encoded string into dictionary and index parts using the separator, unescape dictionary words, split indices, and reconstruct the original list by looking up indices in the dictionary.

5. Test edge cases and justify choices

Test with empty list, single word, repeated words, words containing delimiters, and large inputs. Explain why the chosen delimiter and escaping are robust and efficient.

Key Points to Mention

  • Choice of delimiter: use a character unlikely to appear in input (e.g., non-printable) to minimize escaping overhead.
  • Escaping mechanism: backslash escaping or length-prefixing to handle words containing the delimiter.
  • Dictionary construction: preserve order of first occurrence for deterministic encoding.
  • Index representation: use integers, consider variable-length encoding for space efficiency.
  • Edge cases: empty list, empty strings, words with delimiters, large dictionaries.
  • Time and space complexity: O(n) for encoding/decoding, space proportional to unique words plus indices.

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