Serialize and Deserialize a Binary Tree

Question

## 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
- Adobe specifically asks: how do you ensure your deserializer is robust to corrupted input?