has_cycle.py

# 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:
        slow, fast = head, head
        while slow and fast and fast.next:
            slow = slow.next
            fast = fast.next.next
            if slow == fast:
                return True
        return False


if __name__ == "__main__":
    n1 = ListNode(3)
    n2 = ListNode(2)
    n3 = ListNode(0)
    n4 = ListNode(-4)
    n1.next = n2
    n2.next = n3
    n3.next = n4
    n4.next = n2

    print(Solution().hasCycle(n1))