0606-construct-string-from-binary-tree.py

# 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:
        res = ""
        if not root: return res
        res += str(root.val)
        if root.left:
            res += '(' + self.tree2str(root.left) + ')'
        if not root.left and root.right:
            res += '()'
        if root.right:
            res += '(' + self.tree2str(root.right) + ')'
        return res