1.开辟字典，有额外空间
class Solution(object):
def hasCycle(self, head):
"""
:type head: ListNode
:rtype: bool
"""
dict1={}
if not head or not head.next:
return False
count=-1
while head and head not in dict1:
count+=1
dict1[head]=count
if head.next:
head = head.next
else:
return False
return True
2.快慢指针，没有额外空间
class Solution(object):
def hasCycle(self, head):
"""
:type head: ListNode
:rtype: bool
"""
if not head or not head.next:
return False
quick=slow=head
# 下边这个循环不用担心死循环，因为只要出现环，就会被if语句捕捉，不会永远转下去的
# 若无环，当快指针遍历到最后时，就会结束循环
while quick.next and quick.next.next:
quick=quick.next.next
slow=slow.next
if quick==slow:
return True
return False
