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This is a hard difficulty Coding & Algorithms question, commonly asked during Technical Screen rounds at Amazon.

What role is this question designed for?

This question is commonly asked for Software Engineer candidates at Amazon during technical interviews.

Serialize and deserialize a binary tree

Quick Overview

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.

Quick Overview

Serialize and deserialize a binary tree

Company: Amazon

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: hard

Interview Round: Technical Screen

## 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 1. `deserialize(serialize(root))` must produce a tree structurally identical to `root` with the same node values. 2. 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 3. 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.

Quick Answer: 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.

Serialize a level-order tree with null markers, deserialize it, and return the normalized level-order representation.

Constraints

Inputs are Python literals matching the function signature.
Return a deterministic exact-match value.

Examples

Input: ([1,2,3,None,None,4,5],)

Expected Output: [1, 2, 3, None, None, 4, 5]

Explanation: Round trip with holes.

Input: ([],)

Expected Output: []

Explanation: Empty tree.

Input: ([1,None,2,None,None,3],)

Expected Output: [1, None, 2, None, None, 3]

Explanation: Sparse tree.

Hints

Use deterministic tie-breaking for prompts with multiple valid outputs.
For design-style APIs, simulate operations with explicit inputs.

Last updated: Jun 27, 2026

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