题目分析见这里
class Solution:
# @param head, a ListNode
# @return a list node
def detectCycle(self, head):
if None == head or None == head.next:
return None
pfast = head
pslow = head
#找第一次相遇的点，若存在环路，则肯定会相遇
while pfast and pfast.next:
pfast = pfast.next.next
pslow = pslow.next
if pslow == pfast: break
if pslow != pfast:
return None
#pfast从头开始，则下次相遇的点就是循环开始的点
pfast = head
while True:
if pfast == pslow:
return pfast
pfast = pfast.next
pslow = pslow.next