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

用DFS的方法，用stack保存每个节点，以及从根节点到该节点所有的数。

如果这个当前节点是叶子节点，则用sum = val+sum*10去计算这个读数。

最后将所有读数加到一起即可。

class Solution:    def sumNumbers(self, root: TreeNode) -> int:        if root is None:return 0        stack=[(root, [root.val])]        res=0        while stack:            node, vallist=stack.pop(0)            if node.left is None and node.right is None:                sum=0                for val in vallist:                     sum = val+sum*10                res+=sum            if node.left:                stack.append((node.left, vallist+[node.left.val]))            if node.right:                stack.append((node.right, vallist+[node.right.val]))        return res

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