Deserialize example
Input
'1,2,3,null,4,5'
Output
[1, 2, 3, None, 4, 5]
Reconstruct the tree and represent it as a level-order list with None for missing children.
Full lesson preview
Deserialize a binary tree
Reconstruct a binary tree from its level-order string representation.
Python practice19 minTrees & Binary TreesIntermediateLast updated March 29, 2026
Problem statement
Given a string produced by the serialize function (level-order, comma-separated tokens, 'null' for missing nodes), implement deserialize(data) that reconstructs and returns the original binary tree root. An empty string corresponds to an empty tree (return None).
Task
Implement deserialize to parse the serialization format and rebuild the original binary tree structure.
Examples
Input format
A string encoding a tree produced by serialize (or an empty string).
Output format
A TreeNode root. For testing, use tree_to_list(root) to get a comparable list representation.
Constraints
Input tokens are integers or 'null'. The tree can be empty. Aim for O(n) time.
Samples
Sample 1
Input
'1,2,3,4,5,6,7'
Output
[1, 2, 3, 4, 5, 6, 7]
Perfect binary tree reconstructs with all nodes present.