数据结构与算法（Python）（四）—— 顺序表和链表（4）单向循环链表

4. 单向循环链表

单向循环链表是单链表的一个变形，链表中最后一个节点的next域不再为None，而是指向链表的头节点。

4.1 节点实现

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

4.2 链表实现

• 单向循环链表可执行的操作
  is_empty() 判断链表是否为空
  length() 返回链表的长度
  travel() 遍历
  add(item) 在头部添加一个节点
  append(item) 在尾部添加一个节点
  insert(pos, item) 在指定位置pos添加节点
  remove(item) 删除一个节点
  search(item) 查找节点是否存在
• 代码实现

class SingleCycleLinkList(object):
    def __init__(self, node=None):
        self.__head = node
        if node:
            node.next = node

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

    def length(self):
        """链表长度"""
        if self.is_empty():  # 判断链表是否为空
            return 0
        cursor = self.__head
        count = 1
        while cursor.next != self.__head:
            count += 1
            cursor = cursor.next
        return count

    def travel(self):
        """遍历整个链表"""
        if self.is_empty():
            return
        cursor = self.__head
        while cursor.next != self.__head:
            print(cursor.elem, end='\t')
            cursor = cursor.next
        # 退出循环，cursor指向尾结点，但尾结点的元素未打印
        print(cursor.elem)
        print("")

    def add(self, item):
        """链表头部添加元素"""
        node = Node(item)
        if self.is_empty():
            self.__head = node
            node.next = node
        else:
            cursor = self.__head
            while cursor.next != self.__head:
                cursor = cursor.next
            # 退出循环，cursor指向尾结点
            node.next = self.__head
            self.__head = node
            cursor.next = node

    def append(self, item):
        """链表尾部添加元素"""
        node = Node(item)
        if self.is_empty():
            self.__head = node
            node.next = node
        else:
            cursor = self.__head
            while cursor.next != self.__head:
                cursor = cursor.next
            # 退出循环，cursor指向尾结点
            cursor.next = node
            node.next = self.__head


    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)
            prior = self.__head
            count = 0
            while count < pos-1:
                prior = prior.next
                count += 1
            # 当循环退出后，prior指向pos-1的位置
            node.next = prior.next
            prior.next = node

    def remove(self, item):
        """删除节点"""
        if self.is_empty():
            return
        cursor = self.__head
        prior = None
        while cursor.next != self.__head:
            if cursor.elem == item:
                # 先判断是否是头节点
                if cursor == self.__head:
                    # 头节点
                    # 先找尾结点
                    rear = self.__head
                    while rear.next != self.__head:
                        rear = rear.next
                    # 达到尾结点
                    self.__head = cursor.next
                    rear.next = self.__head
                else:
                    # 中间节点
                    prior.next = cursor.next
                return
            else:
                prior = cursor
                cursor = cursor.next
        # 退出循环，cursor指向尾部
        # 尾结点
        if cursor.elem == item:
            if prior:
                prior.next = cursor.next
            else:
                self.__head = None

    def search(self, item):
        """查找节点是否存在"""
        if self.is_empty():
            return False
        cursor = self.__head
        while cursor.next != self.__head:
            if cursor.elem == item:
                return True
            else:
                cursor = cursor.next
        # 退出循环，cursor指向尾结点
        if cursor.elem == item:
            return True
        return False