Amazon·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

Amazon SWE coding round with a string parsing problem that looked simple but had enough edge cases to trip you up if you weren't careful.

Questions Asked (1)

Write a function that decodes a Morse code string into plain English. Letters are separated by two spaces, words by a single space, and you need to handle invalid Morse sequences gracefully.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The spacing convention tripped me up at first.

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AI HintsAI Generated

Suggested Approach

First, clarify the encoding rules and edge cases (e.g., empty input, invalid sequences). Then, design a solution that splits the input into words and letters, maps Morse codes to characters using a lookup table, and handles invalid sequences by either skipping or replacing them. Finally, discuss trade-offs and test with examples.

Pro tip: Mention that you would validate the input format and consider using a reverse mapping for efficiency, and discuss how to handle ambiguous cases like consecutive spaces.

1. Clarify Requirements

Ask questions to confirm the encoding rules: letters separated by two spaces, words by one space, and how to handle invalid Morse sequences (e.g., skip, replace with placeholder, or throw error).

2. Design the Algorithm

Outline a plan: split the input into words by single spaces, then split each word into letters by double spaces, and map each Morse code to a character using a pre-built dictionary.

3. Handle Edge Cases

Consider empty input, leading/trailing spaces, multiple consecutive spaces, and invalid Morse codes. Decide on a consistent error-handling strategy.

4. Implement and Test

Write clean code with clear variable names, and test with various inputs including valid, invalid, and edge cases to ensure correctness.

5. Discuss Trade-offs

Talk about time and space complexity, and alternative approaches (e.g., using a trie for decoding) and their pros and cons.

Key Points to Mention

Use a hash map (dictionary) for O(1) lookup of Morse code to character.
Splitting strategy: first split by single space to get words, then split each word by double space to get letters.
Handle invalid Morse sequences by either skipping, replacing with a placeholder (e.g., '?'), or throwing an exception, and justify your choice.
Consider edge cases: empty string, multiple spaces, leading/trailing spaces, and invalid characters.
Time complexity: O(n) where n is the length of the input string; space complexity: O(n) for the output.
Mention that you would write unit tests to cover various scenarios.

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