Python链表之单向循环链表

class Node(object):
    """单链表的节点"""
    def __init__(self, item):
        # item存放数据元素
        self.item = item
        # next是下一个节点的标识
        self.next = None


class SingleCycleList(object):
    """单向循环链表"""
    def __init__(self, node=None):
        self._head = node
        if node:  # 如果node不为空，则还需要将node的next指向node
            node.next = node

    def is_empty(self):
        """判断链表是否为空"""
        return self._head == None

    def length(self):
        """链表长度"""
        if self.is_empty():
            return 0
        # cur游标，用来移动遍历节点
        cur = self._head
        # count记录数量
        count = 1
        while cur.next != self._head:
            count += 1
            cur = cur.next
        return count

    def travel(self):
        """遍历链表"""
        if self.is_empty():
            return
        # cur游标，用来移动遍历节点
        cur = self._head
        while cur.next != self._head:
            print(cur.item, end=" ")
            cur = cur.next
        # 退出循环时，cur指向尾节点，但是尾节点的元素未打印
        print(cur.item)

    def add(self, item):
        """头部添加元素，头插法"""
        node = Node(item)
        if self.is_empty():
            self._head = node
            node.next = node
        else:
            cur = self._head
            while cur.next != self._head:
                cur = cur.next
            # 退出循环时，cur指向尾节点
            node.next = self._head
            self._head = node
            cur.next = node

    def append(self, item):
        """尾部添加元素，尾插法"""
        node = Node(item)
        if self.is_empty():
            self._head = node
            node.next = node
        else:
            # cur游标，用来移动遍历节点
            cur = self._head
            while cur.next != self._head:
                cur = cur.next
            node.next = cur.next
            cur.next = node

    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:
            pre = self._head
            count = 0
            while count < (pos - 1):
                count += 1
                pre = pre.next
            # 当循环退出后，pre指向pos-1位置
            node = Node(item)
            node.next = pre.next
            pre.next = node

    def remove(self, item):
        """删除节点"""
        if self.is_empty():
            return
        cur = self._head
        pre = None
        while cur.next != self._head:
            if cur.item == item:
                # 先判断此节点是否是头节点
                if cur == self._head:
                    # 头节点的情况，需要先找尾节点
                    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
            else:
                pre = cur
                cur = cur.next
        # 退出循环时，cur指向尾节点
        if cur.item == item:
            if cur == self._head:
                # 链表只有一个节点
                self._head = None
            else:
                pre.next = cur.next # 等同于prev.next = self._head

    def search(self, item):
        """链表查找节点是否存在，并返回True或者False"""
        if self.is_empty():
            return  False
        cur = self._head
        while cur.next != self._head:
            if cur.item == item:
                return True
            else:
                cur = cur.next
        # 退出循环时，cur指向尾节点
        if cur.item == item:
            return True
        return False


if __name__ == "__main__":
    ll = SingleCycleList()
    print(ll.is_empty())
    print(ll.length())
    ll.add(1)
    print(ll.is_empty())
    print(ll.length())
    ll.append(2)
    ll.append(3)
    ll.append(4)
    ll.travel()
    ll.remove(1)
    ll.travel()
    ll.remove(4)
    ll.travel()
    ll.insert(-1, 1)
    ll.travel()
    ll.insert(6, 4)
    ll.travel()
    res = ll.search(1)
    print(res)