Amazon·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

May 2026Remote

Summary

Amazon SWE round focused entirely on tree serialization and deserialization. Pretty involved question with a lot of back-and-forth on design choices, complexity, and edge cases throughout. Not the kind of round where you just code quietly and move on.

Questions Asked (1)

Design an algorithm to serialize a binary tree into a string and deserialize it back to the original structure. Your encoding format is up to you, but the round-trip must reconstruct the tree exactly.

Algorithms & Data StructuresTechnical Trade-offsSystem Design

Author's notes

I went with preorder traversal and null markers, which felt safe.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then propose a preorder traversal with null markers as a simple and robust solution. Walk through the serialization and deserialization logic, analyze complexity, and discuss trade-offs with alternative approaches like level-order or size-prefixed encoding.

Pro tip: Mention that using a delimiter and null marker avoids ambiguity, and that deserialization can be done in a single pass with an index pointer or iterator. Also, note that the format should be platform-independent and handle large trees efficiently.

1. Clarify Requirements

Ask about constraints: tree size, node values, whether the string must be human-readable, and if there are memory or performance limits. Confirm that the tree structure (including null children) must be preserved exactly.

2. Choose an Encoding Strategy

Select a traversal order (e.g., preorder) and define a format with delimiters and a null marker. Explain why this format is unambiguous and easy to parse.

3. Implement Serialization

Recursively traverse the tree, appending node values and null markers separated by a delimiter. Ensure the output is a single string.

4. Implement Deserialization

Split the string by the delimiter and use a pointer or queue to reconstruct the tree recursively, consuming tokens in the same order as serialization.

5. Analyze and Discuss Trade-offs

State time and space complexity (O(n)), and compare with alternatives like level-order or size-prefixed encoding. Mention edge cases (empty tree, skewed tree) and potential optimizations.

Key Points to Mention

Use a delimiter (e.g., comma) and a null marker (e.g., '#') to represent missing children unambiguously.
Preorder traversal with null markers allows reconstruction without storing additional structural information.
Deserialization can be done in O(n) time using a queue or index pointer to consume tokens sequentially.
The format should be robust to node values containing the delimiter (e.g., escape or use a different delimiter).
Consider space efficiency: the string length is proportional to the number of nodes plus null markers.
Discuss alternative approaches like level-order traversal or encoding subtree sizes, and their trade-offs.

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