给定一个链表，判断它是否有环

 1 """
 2 Definition of ListNode
 3 class ListNode(object):
 4 
 5     def __init__(self, val, next=None):
 6         self.val = val
 7         self.next = next
 8 """
 9 class Solution:
10     """
11     @param head: The first node of the linked list.
12     @return: True if it has a cycle, or false
13     """
14     def hasCycle(self, head):
15         # write your code here
16         slow=head
17         fast=head
18         while fast!=None and fast.next!=None:
19             slow=slow.next
20             fast=fast.next.next
21             if slow==fast:
22                 break
23         if fast==None or fast.next==None:
24             return False
25         else:
26             return True