LeetCode 129. 求根节点到叶节点数字之和

class Solution:
    def sumNumbers(self, root: Optional[TreeNode]) -> int:
        self.total = 0
        if root is None:
            return ans

        def travel(node, current):
            # pre-condition: node is not None
            current_update = f"{current}{node.val}"

            # if is leaf node, add current to total
            if node.left is None and node.right is None:
                self.total += int(current_update)
            if node.left:
                travel(node.left, current_update)
            if node.right:
                travel(node.right, current_update)
        travel(root, "")
        return self.total