给定链表头节点 head，判断链表中是否有环。若有环返回 true，否则返回 false。
class Solution:
def hasCycle(self, head):
if not head or not head.next:
return False
slow = fast = head
while fast and fast.next:
slow = slow.next
fast = fast.next.next
if slow == fast:
return True
return False