How do you serialize and deserialize a binary tree?

Serialization converts tree to string; deserialization reconstructs it. Use pre-order traversal with null markers. Serialize: recursively write node value (or 'null' for null nodes), separated by delimiters. Deserialize: split string into tokens, use recursion - take next token, if 'null' return null, else create node with value and recursively build left and right children. Both operations are O(n) time and space.