Serialize and Deserialize a Binary Tree

Problem

Design an algorithm to serialize a binary tree to a string and deserialize it back to the original tree structure.

Example

Tree:    1
        / \
       2   3
          / \
         4   5

Serialized: "1,2,null,null,3,4,null,null,5,null,null"
Deserialized: same tree

Constraints

• The number of nodes ≤ 10^4
• -1000 ≤ Node.val ≤ 1000

Discussion

• BFS-based serialization produces a more compact format for balanced trees
• DFS pre-order is simpler to implement
• How do you ensure your deserializer is robust to corrupted input?