141. Linked List Cycle

class Solution:
#     def hasCycle(self, head: ListNode) -> bool:
#         seen = {}
#         next_node = head

#         while next_node:
#             if seen.get(next_node) != None:
#                 return True
#             else:
#                 seen[next_node] = 0
#                 next_node = next_node.next

#         return False

    def hasCycle(self, head: ListNode) -> bool:
        if not head:
            return False
        
        slow = head
        fast = head.next
        
        while slow != fast:
            if not fast or not fast.next:
                return False
            else:
                slow = slow.next
                fast = fast.next.next
                
        return True