# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        """
        fast slow pointer
        """
        if not head or not head.next:
            return False
        slow, fast = head, head
        while True:
            try:
                slow = slow.next
                fast = fast.next.next
                if slow is fast:
                    return True
            except Exception:
                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: ListNode) -> bool:
        """
        fast slow pointer
        """
        if not head or not head.next:
            return False
        slow, fast = head, head
        while True:
            try:
                slow = slow.next
                fast = fast.next.next
                if slow is fast:
                    return True
            except Exception:
                return False

# Definition for singly-linked list. # class ListNode: #     def __init__(self, x): #         self.val = x #         self.next = None fast slow pointer