This notebook was prepared by Donne Martin. Source and license info is on GitHub.
slow
, fast
, initialized to the head
slow
and fast
until they meetfast
is incremented twice as fast as slow
fast.next
is None
, we do not have a circular listslow
and fast
meet, move slow
to the head
slow
and fast
one node at a time until they meetComplexity:
%run ../linked_list/linked_list.py
class MyLinkedList(LinkedList):
def find_loop_start(self):
if self.head is None or self.head.next is None:
return None
slow = self.head
fast = self.head
while fast.next is not None:
slow = slow.next
fast = fast.next.next
if fast is None:
return None
if slow == fast:
break
slow = self.head
while slow != fast:
slow = slow.next
fast = fast.next
if fast is None:
return None
return slow
%%writefile test_find_loop_start.py
import unittest
class TestFindLoopStart(unittest.TestCase):
def test_find_loop_start(self):
print('Test: Empty list')
linked_list = MyLinkedList()
self.assertEqual(linked_list.find_loop_start(), None)
print('Test: Not a circular linked list: One element')
head = Node(1)
linked_list = MyLinkedList(head)
self.assertEqual(linked_list.find_loop_start(), None)
print('Test: Not a circular linked list: Two elements')
linked_list.append(2)
self.assertEqual(linked_list.find_loop_start(), None)
print('Test: Not a circular linked list: Three or more elements')
linked_list.append(3)
self.assertEqual(linked_list.find_loop_start(), None)
print('Test: General case: Circular linked list')
node10 = Node(10)
node9 = Node(9, node10)
node8 = Node(8, node9)
node7 = Node(7, node8)
node6 = Node(6, node7)
node5 = Node(5, node6)
node4 = Node(4, node5)
node3 = Node(3, node4)
node2 = Node(2, node3)
node1 = Node(1, node2)
node0 = Node(0, node1)
node10.next = node3
linked_list = MyLinkedList(node0)
self.assertEqual(linked_list.find_loop_start(), node3)
print('Success: test_find_loop_start')
def main():
test = TestFindLoopStart()
test.test_find_loop_start()
if __name__ == '__main__':
main()
Overwriting test_find_loop_start.py
%run -i test_find_loop_start.py
Test: Empty list Test: Not a circular linked list: One element Test: Not a circular linked list: Two elements Test: Not a circular linked list: Three or more elements Test: General case: Circular linked list Success: test_find_loop_start