链接：141. 环形链表 - 力扣（LeetCode）

可以使用哈希判断是否有相同节点

 1 # Definition for singly-linked list.
 2 # class ListNode(object):
 3 #     def __init__(self, x):
 4 #         self.val = x
 5 #         self.next = None
 6 
 7 class Solution(object):
 8     def hasCycle(self, head):
 9         """
10         :type head: ListNode
11         :rtype: bool
12         """
13         help_dict = {}
14         while head:
15             if head in help_dict:
16                 return True
17             else:
18                 help_dict[head] = 1
19             head = head.next
20         return False
21