class Solution(object):
def detectCycle(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
if head is None:
return None
if head.next is None:
return None
first = second = head
while second.next and second.next.next:
first = first.next
second = second.next.next
if first == second:
p = head
while first != p:
p = p.next
first = first.next
return p
return None
142. 环形链表 II
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