80. Serialize/Deserialize Binary Tree - State Persistence

You're building a form builder or state management system that needs to persist complex nested state (a tree) to localStorage or send it over an API.

Implement two functions:

• serialize(root) → returns a string representation of the binary tree
• deserialize(data) → reconstructs the original tree from the string

There is no restriction on the encoding format, but it must satisfy:

• deserialize(serialize(root)) reconstructs the same structure and values.
• Must handle null children.

Canonical format for this problem

Use level-order (BFS) values separated by commas, using the literal string "null" for missing nodes.

For stability, trim trailing `null`s in the serialized output.

Example

Tree: [1,2,3,null,null,4,5]

Serialized: "1,2,3,null,null,4,5"

Real-World Context

• Form state: Persist nested form schemas
• API transfer: Send trees over the network
• localStorage: Save/restore UI state
• State hydration: Serialize app state snapshots