class DoublyLinkedBase:
    """ 双向链表 """

    class _Node:
        def __init__(self):
            pass

    def __init__(self):
        pass

    def __len__(self):
        return self._size

    def is_empty(self):
        pass

    def _insert_between(self, e, predecessor, successor):
        pass

    def _delete_node(self, node):
        pass
