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