Leetcode-141Linked List Cycle

141. Linked List Cycle

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?
题解见leetcode142：https://www.jianshu.com/p/2d0c329fcad2，超详细滴ヽ(•̀ω•́ )ゝ

My Solution:

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        s = set('')
        while head:
            if head not in s:
                s.add(head)
            else:
                return True
            head = head.next
        return False