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

from typing import Optional

class ListNode:
    val: int
    next: Optional["ListNode"]

class Solution:
    def hasCycle(self, head: Optional[ListNode]) -> bool:
        first: Optional[ListNode] = head
        second: Optional[ListNode] = head

        while first is not None and \
            first.next is not None and \
            second is not None:

            first = first.next.next
            second = second.next
            if first == second:
                return True
        return False