leetcode -- Linked List Cycle -- 重点

class Solution(object):
def hasCycle(self, head):
"""
:type head: ListNode
:rtype: bool
"""

if not head : return False
slow, fast = head, head

while fast and fast.next and fast != slow:#这里这个条件fast!=slow，使得在一开始就无法进入循环
slow = slow.next
fast = fast.next.next

if fast == None or fast.next == None:
return False
else:
return True

class Solution(object):
def hasCycle(self, head):
"""
:type head: ListNode
:rtype: bool
"""

if not head : return False
slow, fast = head, head

while fast.next and fast.next.next:#这里用fast and fast.next也可以
slow = slow.next
fast = fast.next.next
if slow == fast:
return True

return False