# 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:
seen = set()
while head:
if head in seen:
return True
seen.add(head)
head = head.next
return False
# 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:
if head is None:
return False
slow = head
fast = head.next
while slow != fast:
if fast is None or fast.next is None:
return False
slow = slow.next
fast = fast.next.next
return True
