141. 环形链表

1. 题目链接

2. 解题思路：一个快指针，一个慢指针，如果二者相等了，说明有环。如果快指针为空了，说明没环

3. 代码

   # Definition for singly-linked list.
   # class ListNode:
   #     def __init__(self, x):
   #         self.val = x
   #         self.next = None
   
   class Solution:
       def hasCycle(self, head: Optional[ListNode]) -> bool:
           slow = head
           if slow == None:
               return False
           fast = slow.next
           if fast == None:
               return False
           while slow != fast:
               slow = slow.next
               fast = fast.next
               if fast == None:
                   return False
               fast = fast.next
               if fast == None:
                   return False
           return True