双向链表

# doublelinklist


class Node(object):
    """节点"""

    def __init__(self, item):
        # 记录数据内容
        self.item = item
        # 记录下一个节点
        self.next = None
        # 记录前置节点
        self.pre = None


class DoubleLinkList(object):
    """双向链表"""

    def __init__(self):
        # 记录头节点，所有的操作从头节点开始
        self.__head = None

    def is_empty(self):
        """链表是否为空"""

        # 判断头节点是否为空
        return self.__head is None

    def length(self):
        """链表长度"""

        if self.is_empty():
            return 0

        # 定义count 累计加1
        count = 0
        # 从头到尾遍历
        cur = self.__head
        while cur is not None:
            count += 1
            cur = cur.next
        return count

    def travel(self):
        """遍历整个链表"""

        if self.is_empty():
            return

        # 定义游标，从头往后移动
        cur = self.__head
        while cur is not None:
            print(cur.item, end=" ")
            cur = cur.next
        print()

    def add(self, item):
        """链表头部添加元素"""

        # 创建新节点
        node = Node(item)

        # 判断链表是否为空
        if self.is_empty():
            self.__head = node
            return

        # 让新节点的next指向原来的头
        node.next = self.__head
        # 让原来的头节点的pre指向新节点
        self.__head.pre = node
        # 让head记录新节点
        self.__head = node

    def append(self, item):
        """链表尾部添加元素"""

        # 判断头节点是否为空
        if self.is_empty():
            self.add(item)
            return

        # 从头遍历，找到尾节点
        cur = self.__head
        while cur.next is not None:
            cur = cur.next

        # 当while循环结束，cur指向的是尾节点
        node = Node(item)
        cur.next = node
        # 让新节点的pre指向原来的尾节点
        node.pre = cur

    def insert(self, pos, item):
        """指定位置添加元素"""

        # 健壮性
        if pos <= 0:
            self.add(item)
        elif pos >= self.length():
            self.append(item)
        else:
            # 从头遍历，找到pos前面的节点
            cur = self.__head

            # 定义下标，index记录的是当前cur指向的对象的下标
            index = 0
            while index < (pos - 1):
                index += 1
                cur = cur.next

            # while循环结束，cur执行的是pos前面的节点
            node = Node(item)
            node.next = cur.next
            # 原来位置节点的pre记录节点
            cur.next.pre = node
            cur.next = node
            # 新节点的pre记录pos前面的节点
            node.pre = cur

    def remove(self, item):
        """删除节点"""

        # 从头遍历，找到要删除的节点
        cur = self.__head
        while cur is not None:
            if cur.item == item:
                # 如果cur.pre为空，证明删除的头节点
                if cur.pre is None:
                    self.__head = cur.next
                else:
                    cur.pre.next = cur.next
                    # 如果cur.next为空，证明删掉的是尾节点
                    if cur.next is not None:
                        cur.next.pre = cur.pre
                return
            cur = cur.next

    def search(self, item):
        """查找节点是否存在"""

        if self.is_empty():
            return False

        cur = self.__head
        while cur is not None:
            if cur.item == item:
                return True
            cur = cur.next

        # while循环结束，没找到
        return False


if __name__ == '__main__':
    sll = DoubleLinkList()
    print(sll.is_empty())
    print(sll.length())
    sll.add(3)
    sll.add(2)
    sll.add(1)
    print(sll.is_empty())
    print(sll.length())
    sll.travel()
    sll.add(4)
    sll.travel()
    print(sll.length())
    sll.append(5)
    sll.travel()
    sll.insert(2,6)
    sll.travel()
    sll.insert(4, 7)
    sll.travel()
    sll.insert(-100, 8)
    sll.insert(100, 9)
    sll.travel()
    sll.remove(6)
    sll.remove(8)
    sll.remove(9)
    sll.travel()
    print(sll.search(4))
    print(sll.search(2))
    print(sll.search(5))
    print(sll.search(6))