【Description】
Given a binary tree containing digits from 0-9 only, each root-to-leaf path could represent a number.

An example is the root-to-leaf path 1->2->3 which represents the number 123.

Find the total sum of all root-to-leaf numbers.

Note: A leaf is a node with no children.

Example:

Input: [1,2,3]
1
/
2 3
Output: 25
Explanation:
The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.
Therefore, sum = 12 + 13 = 25.
Example 2:

Input: [4,9,0,5,1]
4
/
9 0
/
5 1
Output: 1026
Explanation:
The root-to-leaf path 4->9->5 represents the number 495.
The root-to-leaf path 4->9->1 represents the number 491.
The root-to-leaf path 4->0 represents the number 40.
Therefore, sum = 495 + 491 + 40 = 1026.

【Idea】
跟求path的題差不多， 只不過這里是num*10+node.val 在每層遞歸里作參數(shù)傳遞罷遼。
不再寫了，木得意思
similar：257。

【Solution】

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def __init__(self):
        self.result = 0

    def sumNumbers(self, root: TreeNode) -> int:
        
        def recursion(root, num):
            if not root:
                return  
            num = num*10+root.val
            if not root.left and not root.right:    # 葉子節(jié)點
                self.result += num
                return

            recursion(root.left, num)   
            recursion(root.right, num)

        recursion(root, 0)
        return self.result