Serialize and Deserialize Binary Tree

Design a codec for a binary tree: a **serialize** step that turns the tree into a single string, and a **deserialize** step that rebuilds the *exact* same tree from that string. A correct codec is a perfect round trip — deserialize(serialize(t)) must equal t.

This sandbox runs one function, so implement the whole round trip in serializeDeserialize(root): serialize root to a string of your own format, then parse that string back into a tree and return it. You may choose any string format (preorder with null markers is a common one) as long as the rebuilt tree matches the original.

The tree is given (and the result is checked) as a level-order array where null marks a missing child.