# Definition for singly-linked list.
# class ListNode(object):
# def __init__(self, x):
# self.val = x
# self.next = None
class Solution(object):
def detectCycle(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
slow=fast=head
while fast and fast.next:
slow=slow.next
fast=fast.next.next
if id(slow)==id(fast):
fast=head
while fast!=slow:
fast=fast.next
slow=slow.next
return slow
return None
