Serialize and deserialize a binary tree

Q: Serialize and deserialize a binary tree

This question evaluates competency in designing robust serialization formats and manipulating binary tree data structures, including preservation of structure and node values, handling nulls, and reasoning about time and space complexity; it sits in the Coding & Algorithms domain (data structures/trees) and requires practical implementation skills alongside conceptual understanding. It is commonly asked to measure a candidate's ability to produce reproducible data representations, handle edge cases such as empty or sparse trees, and reason about performance and correctness for large inputs.

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Question

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Problem

Design two functions to serialize and deserialize a binary tree.

Serialize : convert a binary tree into a single string.
Deserialize : reconstruct the exact same binary tree from that string.

Assume the tree nodes are:

val is an integer (may be negative)
left / right pointers may be null

Requirements

deserialize(serialize(root)) must produce a tree structurally identical to root with the same node values.
Your format must handle:
- empty tree
- missing children (nulls) in arbitrary positions
- large trees (e.g., up to ~10^5 nodes), so time and space complexity matter
Explain your chosen encoding format and analyze complexity.

Example

If the tree is:

1 as root
left child 2
right child 3 with children 4 and 5

Your serialized output could look like (format is up to you):

"1,2,#,#,3,4,#,#,5,#,#"

where # indicates null.

Serialize and deserialize a binary tree

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