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:
        seen = set()
        node = head
        while node:
            if node in seen:
                return True
            seen.add(node)
            node = node.next
        return False