Serialization method for 297. Serialize and deserialize a binary tree

297_serialize.py

def serialize(self, root) -> str:
    queue, serialize = [root], ""

    while queue:
        next_level = []
        while queue:
            node = queue.pop(0)

            serialize += str(node.val) + "," if node else "null" + ","

            if node: next_level.extend([node.left, node.right])

        queue = next_level if any(next_level) else []

    return serialize[:-1]