题解 | #判断链表中是否有环#

# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

#
# 
# @param head ListNode类 
# @return bool布尔型
#
class Solution:
    def hasCycle(self , head: ListNode) -> bool:
        if head==None:
            return False
        s=set()
        s.add(head)
        while head.next:
            if(head.next in s):
                return True
            else:
                s.add(head.next)
            head=head.next          
        return False

set判重即可，你用快慢指针写也行