我需要帮助从二叉树中构造带括号的字符串

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def tree2str(root):
    if root is None:
        return ""

    result = str(root.val)

    if root.left is None and root.right is None:
        return result

    if root.left is None:
        result += "()"

    if root.left is not None:
        result += "(" + tree2str(root.left) + ")"

    if root.right is not None:
        result += "(" + tree2str(root.right) + ")"

    return result

# 示例用法
root = TreeNode(1)
root.left = TreeNode(2)
root.right = TreeNode(3)
root.left.right = TreeNode(4)

print(tree2str(root))