leetcode 129. 求根节点到叶节点数字之和

深搜遍历每个节点，记录数字即可

# 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: TreeNode) -> int:
        self.ans = 0

        def dfs(root, num):
            if not root:
                return
            if not root.left and not root.right:
                self.ans += num*10 + root.val
                return
            
            dfs(root.left, num*10 + root.val)
            dfs(root.right, num*10 + root.val)

        dfs(root, 0)
        return self.ans