题目：

题解：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = Noneclass Solution(object):def detectCycle(self, head):""":type head: ListNode:rtype: ListNode"""a=headi=0dic={}while a:if a in dic:return adic[a]=ii+=1a=a.nextreturn None