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

class LinkList:
  def __init__(self,head = None):
    self.head = head

  def get_head_node(self):
    """
    获取头部节点
    """
    return self.head
    
  def append(self,value) :
    """
    从尾部添加元素
    """  
    node = Node(value = value) 
    cursor = self.head 
    if self.head is None:
      self.head = node
    else:  
      while cursor.next is not None:
        cursor = cursor.next
    
 
      cursor.next = node
      if value==4:
        node.next = self.head
  
  def traverse_list(self):
    head = self.get_head_node()
    cursor = head
    while cursor is not None:
      print(cursor.value)
      cursor = cursor.next
    print("traverse_over") 
    
  def hasCycle(self, head):
    """
    :type head: ListNode
    :rtype: bool
    """
    slow=fast=head
    while slow and fast and fast.next:
      slow = slow.next
      fast = fast.next.next
      if slow is fast:
        return True
    return False
  

  
def main():
  l = LinkList()
  l.append(1)
  l.append(2)
  l.append(3)
  l.append(4)
  head = l.get_head_node()
  print(l.hasCycle(head))
  #l.traverse_list()


if __name__ == "__main__":
  main()