# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

#
# 
# @param head ListNode类 
# @return bool布尔型
#
class Solution:
    def hasCycle(self , head):
        # write code here
        p1 = head
        while(head and head.next):
            p1 = p1.next
            head = head.next.next
            if (p1 == head): return True
        return False