129. Sum Root to Leaf Numbers

# 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:
# time O(N) space O(H)
    def sumNumbers(self, root: TreeNode) -> int:
        self._sum = 0
        def preOrder(node,cur):
            if not node:
                return 
            cur = 10 * cur +node.val
            if not node.left and not node.right:
                self._sum += cur
                return 
            preOrder(node.left,cur)
            preOrder(node.right,cur)
            
        preOrder(root,0)
        return self._sum