題目描述：

方法一：

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        while head:
            if head.val == "daafas":
                return True
            else:
                head.val = "daafas":
            head 
 
= head.next
        return False

法二：判斷某個元素是否在集合中只有O(1),

class Solution(object):
    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        save = set()
        cur = head 
        while cur is not None:
            if cur in save:
                
 
return True
            else:
                save.add(cur)
                cur = cur.next
        return False 

leetcood學習筆記-141-環形列表