Linked list cycle detection. Python solution for https://www.hackerrank.com/challenges/detect-whether-a-linked-list-contains-a-cycle

cycle_detection.py

"""
Detect a cycle in a linked list. Note that the head pointer may be 'None' if the list is empty.

A Node is defined as:

    class Node(object):
        def __init__(self, data = None, next_node = None):
            self.data = data
            self.next = next_node
"""
import unittest

class Node(object):
    def __init__(self, data = None, next_node = None):
        self.data = data
        self.next = next_node

    def setNext(self, next_node = None):
        self.next = next_node


def has_cycle(head):
    if head == None:
        return False
    
    tortoise = hare = head
    
    while(tortoise or hare or hare.next):
        
        if hare.next == None:
            return False
        
        if tortoise == hare.next or tortoise == hare.next.next:
            return True
        
        tortoise = tortoise.next
        hare = hare.next.next

    return False

class TestStringMethods(unittest.TestCase):

    def test_loop_empty(self):
        self.assertEqual(has_cycle(None), False)

    def test_loop(self):
        self.assertEqual(has_cycle(n5), True)

    def test_no_loop(self):
        self.assertEqual(has_cycle(m5), False)


n1 = Node(1, None)
n2 = Node(2, n1)
n3 = Node(3, n2)
n4 = Node(4, n3)
n5 = Node(5, n4)

n2.setNext(n4) # Loop

m1 = Node(1, None)
m2 = Node(2, m1)
m3 = Node(3, m2)
m4 = Node(4, m3)
m5 = Node(5, m4)


unittest.main()