# 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, head

flag = False # 默认没有还

if not fast:

return None

if not fast.next:

return None

# 先判断有没有环

while fast.next and fast.next.next:

slow = slow.next

fast = fast.next.next

if slow == fast:

flag = True

break

if flag:

p = head

while p != fast:

fast = fast.next

p = p.next

return p

else:

return None