题目地址：https://leetcode.com/problems/sum-root-to-leaf-numbers/description/
题目描述：
Given a binary tree containing digits from 0-9 only, each root-to-leaf path could represent a number.
Anexample is the root-to-leaf path 1->2->3 which represents the number 123.
Find the total sum of all root-to-leaf numbers.
For example,
1
/ \
2 3
The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.
Return the sum = 12 + 13 = 25.
解题方法
这个题和Binary Tree Pathsopen in new window一模一样，前个题是求路径，这个题是把路径按照10的倍数拼接在一起，最后求和即可。
有个技巧就是res = 0当做参数传给函数，那么函数最后的结果不会影响到res，但是如果把res = [0]即可。
代码：
# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution(object):
def sumNumbers(self, root):
"""
:type root: TreeNode
:rtype: int
"""
if root == None:
return 0
res = [0]
self.dfs(root, res, root.val)
return res[0]
def dfs(self, root, res, path):
if root.left == None and root.right == None:
res[0] += path
if root.left != None:
self.dfs(root.left, res, path * 10 + root.left.val)
if root.right != None:
self.dfs(root.right, res, path * 10 + root.right.val)
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