题解 | #判断链表中是否有环#

from sre_compile import FAILURE
# 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:
        n = set()
        if not head:
            return False
        else:
            while head.next:
                if head in n:
                    return True
                else:
                    n.add(head)
                    head = head.next
            return False