606. 根据二叉树创建字符串

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right

class Solution:
    def tree2str(self, root: Optional[TreeNode]) -> str:
        if root is None:
            return ''

        if root.left is not None and root.right is not None:
            return f'{root.val}({self.tree2str(root.left)})({self.tree2str(root.right)})'
        elif root.left is not None:
            return f'{root.val}({self.tree2str(root.left)})'
        elif root.right is not None:
            return f'{root.val}()({self.tree2str(root.right)})'
        else:
            return f'{root.val}'