Apple·Software Engineer·Technical Phone Screen·Senior

Senior

May 2026

Summary

Apple Data Engineer technical screen, just one coding problem but it was a full design question disguised as a coding problem. Took me a while to realize they actually cared about the format choice, not just whether I could write the functions.

Questions Asked (1)

Implement encode and decode functions for a binary tree: encode should serialize a tree to a string, and decode should reconstruct the original tree from that string. Explain your chosen format and make sure empty children are handled so the structure can be fully recovered.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

I went with preorder traversal and used a sentinel value for null nodes, something like '#' separated by commas.

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AI HintsAI Generated

Suggested Approach

Choose a serialization format that explicitly marks null children (e.g., preorder with '#' for nulls) to ensure the tree structure is fully recoverable. Implement encode as a recursive preorder traversal that appends node values and null markers to a string, and decode by parsing the string sequentially to rebuild the tree. Explain why this format is unambiguous and efficient.

Pro tip: Mention that using a delimiter (like comma) and a null marker (like '#') prevents ambiguity with negative numbers or multi-digit values, and discuss how your approach handles edge cases like empty trees or skewed trees.

1. Clarify requirements and constraints

Ask about the tree type (binary, not necessarily BST), allowed characters, and whether the serialized string needs to be human-readable. Confirm that the tree may be empty and that node values can be any integer.

2. Choose a serialization format

Select a format that includes null markers, such as preorder traversal with '#' for null and commas as delimiters. Explain why this format is unambiguous and allows exact reconstruction.

3. Implement encode

Write a recursive function that performs preorder traversal, appending node values and '#' for null children to a list, then join with commas. Ensure the base case handles null nodes.

4. Implement decode

Split the string by commas and use an iterator or index to consume tokens. Recursively build the tree: if token is '#', return null; otherwise create a node and recursively decode left and right subtrees.

5. Analyze complexity and trade-offs

State that both encode and decode run in O(n) time and O(n) space, where n is the number of nodes. Discuss alternative formats (e.g., level-order, parentheses) and their trade-offs in readability and efficiency.

Key Points to Mention

Use of preorder traversal with explicit null markers to preserve structure.
Delimiter choice (e.g., comma) to separate values and avoid ambiguity.
Handling of empty tree (encode returns empty string or '#'; decode returns null).
Time and space complexity: O(n) for both operations.
Recursive implementation details and base cases.
Comparison with other serialization methods (e.g., level-order, JSON-like) and why this is suitable.

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