Leetcode-141环形链表

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """

        cur = head
        res = set()

        while cur:
            node = cur
           
            if node not in res:
                cur = cur.next
                res.add(node)
            else:
                return True
        return False