双向链表

# 创建节点类
class Node():
    def __init__(self,item):
        # 节点数据
        self.item=item
        # 上一个节点地址
        self.pre=None
        # 下一个节点地址
        self.next=None
class DoubleLinkList():
    def __init__(self,node=None):
        self.__head=node
    #判断链表是否为空
    def is_empty(self):
        return self.__head is None
    # 链表长度
    def length(self):
        cur=self.__head
        count=0
        while cur is not None:
            cur=cur.next
            count+=1
        return count
    # 循环链表
    def travel(self):
        cur=self.__head
        while cur is not None:
            print(cur.item,end='')
            cur=cur.next
        print('')
    # 搜索节点
    def search(self,item):
        cur=self.__head
        while cur is not None:
            if cur.item==item:
                return True
            cur=cur.next
        return False
    # 头部增加节点
    def add(self,item):
        node=Node(item)
        node.next=self.__head
        self.__head=node
        if node.next:
            node.next.pre=node
    # 尾部增加节点
    def append(self,item):
        node=Node(item)
        if self.is_empty():
            self.__head=node
        else:
            cur=self.__head
            while cur.next is not None:
                cur=cur.next
            node.pre=cur
            cur.next=node
    # 指定位置插入
    def insert(self,pos,item):
        node=Node(item)
        if pos<=0:
            self.add(item)
        elif pos>=self.length():
            self.append(item)
        else:
            cur=self.__head
            count=0
            while count<pos-1:
                cur=cur.next
                count+=1
            # 顺序赋值，先和上一个节点链接，再和下一个节点链接，然后下一个节点再和本节点链接
            node.pre=cur.pre
            cur.pre.next=node
            node.next=cur
            cur.pre=node
    # 删除节点
    def remove(self,item):
        cur=self.__head
        while cur is not None:
            if cur.item==item:
                if cur==self.__head:
                    self.__head=cur.next
                    if cur.next:
                        self.__head.pre=None
                else:
                    cur.pre.next=cur.next
                    if cur.next:
                        cur.next.pre=cur.pre
                return
            cur=cur.next
if __name__ == '__main__':
    dd=DoubleLinkList()
    for i in range(10):
        dd.append(i)
    dd.travel()
    print(dd.length())
    dd.travel()
    dd.add(1)
    dd.travel()
    dd.insert(4,5)
    dd.travel()
    dd.remove(5)
    dd.travel()
    dd.remove(5)
    dd.travel()