solve

problem

Same as the last problem. We only want to add when there's no left or right node. State is kept by function.

from typing import Optional
# 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 sumNumbers(self, root: Optional[TreeNode]) -> int:
        return self.dfs(root, 0)
    
    def dfs(self, node, currSum) -> int:

        if node == None:
            return 0
        
        if node.left == None and node.right == None:
            return int(str(currSum) + str(node.val))
        
        else:
            currSum = str(currSum) + str(node.val)
            return self.dfs(node.left, currSum) + self.dfs(node.right, currSum)