leetcode：环形链表

方法一：暴力法

class Solution:
    """
    思路：检查一个结点是否已经被访问过
    """
    def hasCycle(self, head: ListNode) -> bool:
        lst = []
        while head != None :        #当head为空结点时表示已经循环过一遍了 
            if head in hashset :
                return True
            else :
                lst.append(head)
                head = head.next
        return False

方法二：快慢指针

class Solution:
    """
    快指针追到慢指针说明有环 ，追不到寿命没有环
    """
    def hasCycle(self, head: ListNode) -> bool:
        fast = head
        slow = head
        while fast and fast.next :
            fast = fast.next.next
            slow = slow.next
            if fast == slow : return True
        return False