chal_141.py

class ListNode(object):
    def __init__(self, x):
        self.val = x
        self.next = None


class Solution(object):
    def has_cycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        slow_pointer = head
        fast_pointer = head

        while fast_pointer and fast_pointer.next:
            slow_pointer = slow_pointer.next
            fast_pointer = fast_pointer.next.next
            if slow_pointer == fast_pointer:
                return True
        return False


# Test cases
sol = Solution()

ll = ListNode(3)
ll.next = ListNode(2)
ll.next.next = ListNode(0)
ll.next.next.next = ListNode(-4)
ll.next.next.next.next = ll.next

assert sol.has_cycle(ll)

ll = ListNode(1)
ll.next = ListNode(2)
ll.next.next = ll.next

assert sol.has_cycle(ll)

ll = ListNode(1)

assert not sol.has_cycle(ll)