https://leetcode.cn/problems/linked-list-cycle/
需要注意的点

1. 快慢指针 ```python

   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
    fast = head
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
        if slow == fast:
            return True
    return False

```