如何创建递归分隔符？

class TreeNode:
    def __init__(self, value):
        self.value = value
        self.children = []

    def add_child(self, child_node):
        self.children.append(child_node)

    def to_string(self):
        result = f"({self.value}"
        for child in self.children:
            result += f" {child.to_string()}"
        result += ")"
        return result

# 创建一个树形结构
root = TreeNode("root")
child1 = TreeNode("child1")
child2 = TreeNode("child2")
grandchild = TreeNode("grandchild")

root.add_child(child1)
root.add_child(child2)
child1.add_child(grandchild)

# 序列化为字符串
serialized_tree = root.to_string()
print(serialized_tree)  # 输出: (root (child1 (grandchild)) (child2))