class Node:
    def __init__(self, data):
        self.data = data
        self.visited = False
        self.children = []

    def is_internal_external(self):
        if self.children:
            return True
        return False

    def get_children(self):
        return self.children



    def __str__(self):
        return self.data
    def __repr__(self):
        return self.__str__()

def traverse (node, stack = None):
    if stack is None:
        stack = []

    if node.children is not None:
        for child in node.children:
            if not child.visited:
                child.visited = True
                stack.append(child)
                print(child.data)
                traverse(child)
    else:
        if len(stack) > 0:
            traverse(stack[len(stack) - 1])
            stack.pop()

root = Node(6)
node_two = Node(2)
node_seven = Node(7)
node_one = Node(1)
node_four = Node(4)
node_three = Node(3)
node_five = Node(5)
node_six = Node(6)
node_nine = Node(9)

root.children = [node_two, node_seven, node_one]
node_two.children = [node_four, node_three]
node_seven.children = [node_five, node_six]

traverse(root)