双链表的实现出错

Question

在理解了算法之后，我在Python中为双链表编写了一个程序。但是只有功能的某些部分正在工作。这是我的代码

class Node(object):

    def __init__(self,d):
        self.next_node = None
        self.prev_node = None
        self.data = d

class DoublyLinkedList(object):

    def __init__(self):
        self.head = None
        self.tail = None
        self.size = 0

    def add(self,d):
        new_node = Node(d)
        if self.tail:
            self.tail.next_node = new_node
            new_node.next_node = None
            new_node.prev_node = self.tail
        else:
            self.head = new_node
            self.tail = new_node
            new_node.prev_node = None
        self.size+=1

    def addBeg(self,d):
        new_node = Node(d)
        current_node = self.head
        current_node.prev_node = new_node
        new_node.prev_node = None
        new_node.next_node = current_node
        self.head = new_node
        self.size+=1

    def add_at(self,d,index):
        new_node = Node(d)
        previous_node = None
        current_node = self.head
        i = 0
        while i<index and current_node:
            previous_node = current_node
            current_node = current_node.next_node
            i+=1
        #once it reaches the desired index
        if i==index:
            previous_node.next_node = new_node
            new_node.prev_node = previous_node
            new_node.next_node = current_node
            current_node.prev_node = new_node
            self.size+=1
            return True
        else:
            return False                #list ain't too long


    def remove(self,d):
        previous_node = None
        current_node = self.head
        while current_node:
            if current_node.data == d:
                if previous_node:  #the node is somewhere in between
                    previous_node.next_node = current_node.next_node
                    current_node.next_node.prev_node = previous_node
                else:
                    #it is the first node
                    self.head = current_node.next_node
                    current_node.next_node.prev_node = None
                self.size -= 1
                return True

            previous_node = current_node
            current_node = current_node.next_node
        return False

    def search(self,d):
        current_node = self.head
        while current_node:
            if current_node.data == d:
                return True
            current_node = current_node.next_node
        return False

    #traverse through the linked list and store the elements in a list
    def to_list(self):
        lis = []
        current_node = self.head
        while current_node:
            lis.append(current_node.data)
            current_node = current_node.next_node
        return lis

dll = DoublyLinkedList()

当我尝试使用add函数插入多个元素时，它表现得很奇怪。例如，如果我尝试添加三个元素，它只显示第一个和最后一个

ll.add(40)
ll.add(50)
ll.add(60)
print(ll.to_list())

我得到输出为[40,60]

第二个问题是如果我删除了其他内容而只保留一个元素

#removed all other elements
ll.add(70)

我得到'没有属性'错误。发生了什么事？

Answer 1

添加功能时出现问题，忘记更新 self.tail 。

def add(self,d):
    new_node = Node(d)
    if self.tail:
        self.tail.next_node = new_node
        new_node.next_node = None
        new_node.prev_node = self.tail
        self.tail=new_node #change is here
    else:
        self.head = new_node
        self.tail = new_node
        new_node.prev_node = None
    self.size+=1

删除最后一个节点的另一个问题。

def remove(self,d):
    previous_node = None
    current_node = self.head
    while current_node:
        if current_node.data == d:
            if previous_node:  #the node is somewhere in between
                #print(d)
                previous_node.next_node = current_node.next_node
                if current_node.next_node is None: #deleting last node
                    current_node.prev_node=None
                else:
                    current_node.next_node.prev_node = previous_node
            else:
                #it is the first node
                self.head = current_node.next_node
                current_node.next_node.prev_node = None
            self.size -= 1
            return True

        previous_node = current_node
        current_node = current_node.next_node
    return False

Working Code.希望这会有所帮助。