链表：单向循环链表

函数说明：

• 初始化；
• 判空；
• 求长度；
• 插入节点：头插、尾插、随机插入；
• 遍历节点；
• 删除节点；
• 查找节点（搜索节点）；

代码：

class Node(object):
    def __init__(self, elem):
        self.elem = elem
        self.next = None
        
class SingleLinkList(object):
    """单向循环链表"""
    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 == None
    
    def length(self):
        if self.is_empty():
            return 0
        # 游标用来遍历节点
        cur = self.__head
        # 节点计数器，初始值影响while循环的判断条件
        counter = 1
        while cur.next != self.__head :
            counter += 1
            cur = cur.next
        return counter
            
    def travel(self):
        if self.is_empty():
            return 
        cur = self.__head
        while cur.next != self.__head:
            print(cur.elem)
            cur = cur.next
#         退出循环，cur指向尾节点，但尾节点的元素未打印
        print(cur.elem)
        
    def add(self, item):
        """链表头部添加元素"""
#         print("=====头插法=====")
        node = Node(item)
        if self.is_empty():
            self.__head = node
            node.next = node
        else:
    #         遍历寻找尾节点
            cur = self.__head
            while cur.next != self.__head:
                cur = cur.next
    #           退出循环，cur指向的就是尾节点
            node.next = self.__head
            self.__head = node
            cur.next = node
        
    
    def append(self, item):
        """链表尾部添加元素"""
        node = Node(item)
#         print("=====尾插法=====")
        if self.is_empty():
            self.__head = node
            node.next = node
        else:
            cur = self.__head
            while cur.next != self.__head:
                cur = cur.next
#             node.next = cur.next
            node.next = self.__head
            cur.next = node
    
    def insert(self, pos, item):
        """
        指定位置添加元素
        @pos: 从0开始 例：insert(2,100)        
        因为不涉及尾部，所以不需要修改代码。
        """
        if pos <= 0 :
            self.add(item)
        elif pos > (self.length() - 1):
            self.append(item)
        else:
            node = Node(item)
            pre = self.__head
            count = 0
            while count < (pos - 1):
                pre = pre.next
                count += 1
    #             当循环退出后，pre指向pos-1位置
            node.next = pre.next
            pre.next = node
        
    def search(self, item):
        """查找节点是否存在"""
        if self.is_empty(): # 判断特殊情况
            return False
        cur = self.__head
        while cur.next != self.__head: # 修改条件，但丢了尾节点
            if cur.elem == item:
                return True
            else:
                cur = cur.next
#       退出循环，cur指向的就是尾节点
        if cur.elem == elem:
            return True
        return False
    
#     难度系数高
    def remove(self, item):
        if self.is_empty():
            return
        cur = self.__head
        pre = None
        while cur.next != self.__head: # 修改条件
            if cur.elem == item:
                # 先判断此节点是否为头节点
                if cur == self.__head:
                    # 头节点
                    # 找尾节点
                    rear = self.__head
                    while rear.next != self.__head:
                        rear = rear.next
                    self.__head = cur.next
                    rear.next = self.__head
                else:
                    # 中间节点
                    pre.next = cur.next
                return # break
            else: # 游标移动
                pre = cur
                cur = cur.next
            # 退出循环，cur指向尾节点
        if cur.elem == item:
            if cur == self.__head:
                # 链表只有一个节点
                self.__head = None
            else:
#               pre.next = cur.next
                pre.next = self.__head