Question

事实证明是错误的，因为超过了时间限制，但是我已经提出了StopIteration ...

我认为我在迭代部分做错了什么，但是很难发现错误。测试输出保持运行，甚至打印出None值。它是怎么发生的？

class LinkedListIterator:
    def __init__(self, head):
        self.__current = head.get_next()

    def __iter__(self):
        return self

    def __next__(self):
        if self.__current == None:
            raise StopIteration
        else:
            item = self.__current.get_data()
            self.__current = self.__current.get_next()
            return item

这些是我用来运行程序的输入：

my_list = LinkedListDLL()
my_list.add_to_head(1)
print("Contents:", end=" ")
for node in my_list:
    print(node, end=" ")
print()

Answer 1

此代码用于在到达列表开头时停止迭代。

    if self.__current == None:
        raise StopIteration

但是，您用与NodeDLL不同的None对象表示头部。

您可以保留对头部的引用，然后对照它进行检查：

class LinkedListIterator:
    def __init__(self, head):
        self._head = head
        self._current = head.get_next()

    def __iter__(self):
        return self

    def __next__(self):
        if self._current is self._head:
            raise StopIteration
        else:
            item = self._current.get_data()
            self._current = self._current.get_next()
            return item

Answer 2

您要实现的是MutableSequence的API，并实现了双向链接列表。

要在Python中做到这一点，您应该依靠collections.abc，它可以指导您完成所有必需方法的实现过程。

通过示例，链接列表实际上是从MutableSequence继承的类。

from collections.abc import MutableSequence

class LinkedList(MutableSequence):
    pass

ll = LinkedList()

在实例化具有一些尚未编写的抽象方法的类时，您将获得一个TypeError，它将指导您完成需要实现的方法。

TypeError: Can't instantiate abstract class LinkedList with abstract methods __delitem__, __getitem__, __len__, __setitem__, insert

尤其要注意，list或链接列表不是迭代器，而是可迭代。这意味着__iter__方法不应返回self并依赖__next__，而应在链表的内容上返回一个全新的迭代器。

换句话说，您只能通过 iterator 进行一次迭代，并通过和 iterable 进行多次。

全面实施

事实证明，我已经完全实现了以这种方式实现的双链表。你可以看看。

from collections.abc import MutableSequence

class LinkedList(MutableSequence):
    class _Node:
        def __init__(self, value, _next=None, _last=None):
            self.value, self._next, self._last = value, _next, _last

        def __str__(self):
            return f'Node({self.value})'

    def __init__(self, iterable=()):
        self.start = None
        self.last = None

        empty = object()
        iterable = iter(iterable)
        first = next(iterable, empty)

        if first is empty:
            return

        current = self._Node(first)
        self.start, self.last = current, current

        for value in iterable:
            new_node = self._Node(value, _last=self.last)
            self.last._next = new_node
            self.last = new_node


    def __len__(self):
        if self.start is None:
            return 0
        else:
            return sum(1 for _ in self)

    def __iter_nodes(self):
        current = self.start

        while current is not None:
            yield current
            current = current._next

    def __reversed_iter_nodes(self):
        current = self.last

        while current is not None:
            yield current
            current = current._last

    def __iter__(self):
        for node in self.__iter_nodes():
            yield node.value

    def __reversed__(self):
        for node in self.__reversed_iter_nodes():
            yield node.value

    def __get_node(self, index):
        if index >= 0:
            for item in self.__iter_nodes():
                if index == 0:
                    return item
                index -= 1
        else:
            for item in self.__reversed_iter_nodes():
                if index == 0:
                    return item
                index += 1
        raise IndexError

    def __getitem__(self, index):
        if index >= 0:
            for item in self:
                if index == 0:
                    return item.value
                index -= 1
        else:
            for item in reversed(self):
                if index == 0:
                    return item.value
                index += 1
        raise IndexError

    def __setitem__(self, key, value):
        self[key].value = value

    def __delitem__(self, key):
        node = self[key]
        if node._last:
            node._last._next = node._next
        if node._next:
            node._next._last = node._last

    def insert(self, index, value):
        if index > len(self):
            self.last = self._Node(value, _last=self.last)
        else:
            where = self.__get_node(index)
            _last = where._last
            new_node = self._Node(value, _next=where, _last=_last)
            if _last:
                _last._next = new_node
            else:
                self.start = new_node

            where._last = new_node

示例

ll = LinkedList(range(1, 5))
print(*ll)

print(*reversed(ll))

ll.insert(2, 'foo')
print(*ll)

输出

1 2 3 4
4 3 2 1
1 2 foo 3 4

Python中的双链表

2 个答案:

全面实施

示例

输出