129.求根到叶子节点数字之和

方法：回溯法思路：    step 1：判空    step 2：回溯

# Definition for a binary tree node.# class TreeNode:#     def __init__(self, x):#         self.val = x#         self.left = None#         self.right = None
class Solution:    def sumNumbers(self, root: TreeNode) -> int:        self.res = 0        if not root:            return self.res         self.dfs(root,0)        return self.res         # 回溯    def dfs(self,root,path):        # step 1：修改        path = path*10+root.val        # step 2:判断终止条件        if not root.left and not root.right:            self.res = self.res+path            return         # step 3：遍历 左子树        if root.left:            self.dfs(root.left,path)        # step 3：遍历 右子树        if root.right:            self.dfs(root.right,path)