Serialize and Deserialize Binary Tree

Implement `serialize(root)` returning a string, and `deserialize(data)` rebuilding the tree, so that the round trip preserves the tree exactly. Pre-order with an explicit marker for every missing child is the shortest correct scheme: the markers are what make the structure unambiguous. Without them, a pre-order sequence alone cannot tell a left child from a right one. Your format is your choice — it just has to survive skewed trees, negative values, and an empty tree.