class NodeIndexHistory(object):
    def __init__(self):
        self._: List[int] = []

    def add(self, node: 'Node'):
        if self.current():
            Node.nodes_storage[node.index].back_str = self.current().back_str
        self._.append(node.index)

    def set_root(self, root_node: 'Node'):
        self._ = [root_node.index]

    def current(self) -> Union['Node', None]:
        if len(self._):
            return Node.nodes_storage[self._[-1]]
        return None

    def can_back(self) -> bool:
        return len(self._) > 1

    def back(self) -> 'Node':
        self._.pop()
        return self.current()

    def __len__(self):
        return len(self._)

    def __repr__(self):
        return '<{cls} nodes_number: {nn}>'.format(cls=self.__class__.__name__, nn=len(self))