python实现双链表

class Node:
    def __init__(self, item):
        self.elem = item
        self.next = None
        self.pre = None


class DoubleLinkList:
    def __init__(self, node=None):
        self._head = node

    def is_empty(self):
        return self._head is None

    def length(self):
        """
        获取长度，同单链表的获取长度
        :return:
        """
        count = 0
        cur = self._head
        while cur is not None:
            count += 1
            cur = cur.next
        return count

    def travel(self):
        """
        遍历，同单链表的遍历
        :return:
        """
        cur = self._head
        while cur is not None:
            print(cur.elem, end=" ")
            cur = cur.next
        print("")

    def add(self, item):
        """
        添加节点
        :param item:
        :return:
        """
        node = Node(item)
        node.next = self._head
        # node.next.pre = node
        # self._head = node
        # 或者答案不为1
        self._head = node
        node.next.pre = node

    def append(self, item):
        """
        :param item:
        :return:
        """
        node = Node(item)
        if self.is_empty():
            self._head = node
        else:
            cur = self._head
            while cur.next is not None:
                cur = cur.next
            cur.next = node
            node.pre = cur

    def insert(self, pos, item):
        if pos <= 0:
            self.add(item)
        elif pos > (self.length() - 1):
            self.append(item)
        else:
            cur = self._head
            count = 0
            while count < pos:
                count += 1
                cur = cur.next
            node = Node(item)
            node.next = cur
            node.pre = cur.pre
            # 断开原来的链接
            cur.pre.next = node
            cur.pre = node

    def search(self, item):
        """
        查找节点
        :param item:
        :return:
        """
        cur = self._head
        while cur is not None:
            if cur.elem == item:
                return True
            else:
                cur = cur.next
        return False

    def delete(self, item):
        cur = self._head
        while cur is not None:
            if cur.elem == item:
                if cur == self._head:
                    self._head = cur.next
                    # 判断链表是否只有一个节点
                    if cur.next:
                        cur.next.pre = None
                else:
                    cur.pre.next = cur.next
                    # 判断是否是结尾的节点
                    if cur.next:
                        cur.next.pre = cur.pre
                break
            else:
                cur = cur.next