链接：142. 环形链表 II - 力扣（LeetCode）

用哈希表，和环形链表[Python刷题记录]-环形链表-链表-简单 - Annetree - 博客园好像一样的，只是输出的值不一样

 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 detectCycle(self, head):
 9         """
10         :type head: ListNode
11         :rtype: ListNode
12         """
13         help_dict = {}
14         while head:
15             if head in help_dict:
16                 return head
17             else:
18                 help_dict[head] = 1
19             head = head.next
20         return None