129. 求根到葉子節點數字之和

# 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):
        """
        :type root: TreeNode
        :rtype: int
        """
        result = 0
        if not root:
            return result

        path = [(0, root)]
        while path:
            pre, node = path.pop()
            if node:
                if not node.left and not node.right :
                    result += pre*10 + node.val
                
                path += [(pre*10 + node.val, node.right), (pre*10 + node.val, node.left)]
            
        return result