129. 求根节点到叶节点数字之和

思路：

找出根节点到叶子节点的路径，然后计算每条路径的数字之和

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def sumNumbers(self, root: TreeNode) -> int:
        self.total=0
        result=[]
        if not root:
            return 0
        def trace(root):
            if not root:
                return
            result.append(root.val)
            trace(root.left)
            if not root.left and not root.right:
                sumvalue=0
                n=len(result)
                for i in range(len(result)):
                    sumvalue *= 10
                    sumvalue+=result[i]
                self.total+=sumvalue
            trace(root.right)
            result.pop()
        trace(root)
        return self.total