path: root/data_structures_and_algorithms/linked-list-cycles.py

import unittest


################################################################################
# Solution
################################################################################
def contains_cycle(x):
    if not x:
        return False
    elif not x.next:
        return False

    a = x
    b = x.next

    while b.next:
        if a == b:
            return True

        a = a.next
        b = b.next.next

    return False


################################################################################
# Tests
################################################################################
class Test(unittest.TestCase):
    class LinkedListNode(object):
        def __init__(self, value, next=None):
            self.value = value
            self.next = next

    def test_linked_list_with_no_cycle(self):
        fourth = Test.LinkedListNode(4)
        third = Test.LinkedListNode(3, fourth)
        second = Test.LinkedListNode(2, third)
        first = Test.LinkedListNode(1, second)
        result = contains_cycle(first)
        self.assertFalse(result)

    def test_cycle_loops_to_beginning(self):
        fourth = Test.LinkedListNode(4)
        third = Test.LinkedListNode(3, fourth)
        second = Test.LinkedListNode(2, third)
        first = Test.LinkedListNode(1, second)
        fourth.next = first
        result = contains_cycle(first)
        self.assertTrue(result)

    def test_cycle_loops_to_middle(self):
        fifth = Test.LinkedListNode(5)
        fourth = Test.LinkedListNode(4, fifth)
        third = Test.LinkedListNode(3, fourth)
        second = Test.LinkedListNode(2, third)
        first = Test.LinkedListNode(1, second)
        fifth.next = third
        result = contains_cycle(first)
        self.assertTrue(result)

    def test_two_node_cycle_at_end(self):
        fifth = Test.LinkedListNode(5)
        fourth = Test.LinkedListNode(4, fifth)
        third = Test.LinkedListNode(3, fourth)
        second = Test.LinkedListNode(2, third)
        first = Test.LinkedListNode(1, second)
        fifth.next = fourth
        result = contains_cycle(first)
        self.assertTrue(result)

    def test_empty_list(self):
        result = contains_cycle(None)
        self.assertFalse(result)

    def test_one_element_linked_list_no_cycle(self):
        first = Test.LinkedListNode(1)
        result = contains_cycle(first)
        self.assertFalse(result)

    def test_one_element_linked_list_cycle(self):
        first = Test.LinkedListNode(1)
        first.next = first
        result = contains_cycle(first)
        self.assertTrue(result)


unittest.main(verbosity=2)