数据结构与算法（Python）（四）—— 顺序表和链表（3）双向链表

3. 双向链表

• “双向链表”或“双面链表”指每个节点有两个链接：一个指向前一个节点，当此节点为第一个节点时，指向空值；而另一个指向下一个节点，当此节点为最后一个节点时，指向空值。
  

3.1 节点实现

class Node(object):
    def __init__(self, elem):
        self.elem = elem
        self.next = None
        self.prev = None

3.2 链表实现

• 双向链表可执行的操作
  is_empty() 链表是否为空
  length() 链表长度
  travel() 遍历链表
  add(item) 链表头部添加
  append(item) 链表尾部添加
  insert(pos, item) 指定位置添加
  remove(item) 删除节点
  search(item) 查找节点是否存在
• 代码实现（代码后方有几种情况的图示解释）

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

    def is_empty(self):
        """链表是否为空"""
        return self.__head is None

    def length(self):
        """链表长度"""
        cursor = self.__head
        count = 0
        while cursor is not None:
            count += 1
            cursor = cursor.next
        return count

    def travel(self):
        """遍历整个链表"""
        cursor = self.__head
        while cursor is not None:
            print(cursor.elem, end='\t')
            cursor = cursor.next
        print("")

    def add(self, item):
        """链表头部添加元素"""
        node = Node(item)
        node.next = self.__head
        self.__head = node
        node.next.prev = node

    def append(self, item):
        """链表尾部添加元素"""
        node = Node(item)
        if self.is_empty():
            self.__head = node
        else:
            cursor = self.__head
            while cursor.next is not None:
                cursor = cursor.next
            cursor.next = node
            node.prev = cursor


    def insert(self, pos, item):
        """指定位置添加元素
        :param pos 从0开始
        """
        if pos <= 0:
            self.add(item)
        elif pos > self.length()-1:
            self.append(item)
        else:
            node = Node(item)
            count = 0
            cursor = self.__head
            while count < pos:
                cursor = cursor.next
                count += 1
            node.next = cursor
            node.prev = cursor.prev
            cursor.prev.next = node
            cursor.prev = node

    def remove(self, item):
        """删除节点"""
        cursor = self.__head
        while cursor is not None:
            if cursor.elem == item:
                # 先判断是否是头节点
                if cursor == self.__head:
                    self.__head = cursor.next
                    if cursor.next is not None:
                        cursor.next.prev = None
                    else:
                        pass  # 只有一个节点
                else:
                    cursor.prev.next = cursor.next
                    if cursor.next is not None:
                        cursor.next.prev = cursor.prev
                    else:
                        pass  # 最后一个节点
                return True
            else:
                cursor = cursor.next

    def search(self, item):
        """查找节点是否存在"""
        cursor = self.__head
        while cursor is not None:
            if cursor.elem == item:
                return True
            else:
                cursor = cursor.next
        return False

• 几种情况的解释

1. 指定位置插入元素
   
2. 删除元素