# Definition for singly-linked list.
# class ListNode(object):
# def __init__(self, x):
# self.val = x
# self.next = None
class Solution(object):
def hasCycle(self, head):
"""
:type head: ListNode
:rtype: bool
"""
if not head or not head.next:
return False
fast=head
slow=head
while(fast.next and fast.next.next):
fast=fast.next.next
slow=slow.next
if slow.val==fast.val:
return True
return False