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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def levelorder(self):
 """Return the nodes in the binary tree using level-order_ traversal. A level-order_ traversal visits nodes left to right, level by level. .. _level-order: https://en.wikipedia.org/wiki/Tree_traversal#Breadth-first_search :return: List of nodes. :rtype: [binarytree.Node] **Example**: .. doctest:: <BLANKLINE> __1 / \\ 2 3 / \\ 4 5 <BLANKLINE> [Node(1), Node(2), Node(3), Node(4), Node(5)] """

        current_nodes = [self]
        result = []

        while len(current_nodes) > 0:
            next_nodes = []
            for node in current_nodes:
                result.append(node)
                if node.left is not None:
                    next_nodes.append(node.left)
                if node.right is not None:
                    next_nodes.append(node.right)
            current_nodes = next_nodes

        return result