Serialize and Deserialize Binary Tree

Design an algorithm to turn a binary tree into a string and back. Implement the Codec class:

• serialize(root) returns the string encoding of the tree.
• deserialize(data) rebuilds the tree from a string produced by serialize and returns its root.

Since the tests check the strings, use this exact format: a preorder traversal (node, then left subtree, then right subtree) with values joined by ,, writing # for every missing child. For example the tree [1,2,3,null,null,4,5] is "1,2,#,#,3,4,#,#,5,#,#", and an empty tree is "#".