https://leetcode.com/problems/linked-list-cycle/

hasCycle.py

class Solution:
    def hasCycle(self, head: Optional[ListNode]) -> bool:
        seen = set()
        
        node = head
        
        while node:
            if node in seen:
                return True
            else:
                seen.add(node)
            node = node.next
        return False

hasCycle2.py

class Solution:
    def hasCycle(self, head: Optional[ListNode]) -> bool:
        
        if not(head and head.next):
            return False
        
        slow , fast = head , head.next
        
        while fast and fast.next:
            if slow == fast:
                return True
            slow = slow.next
            fast = fast.next.next
            
        return False