class Solution(object):
def hasCycle(self, head):
tag = False
#边界条件
if head == None :
return None
#边界条件 单节点无环
if head != None and head.next == None :
return False
#边界条件 单节点有环
if head != None and head.next == head:
return True
fast = head.next;
low = head;
while (fast != low ):
try:
fast = fast.next.next;
low = low.next;
if fast == low:
tag = True
break
except:
break
return tag
