This notebook was prepared by [Donne Martin](http://donnemartin.com). Source and license info is on [GitHub](https://github.com/donnemartin/interactive-coding-challenges).
Complexity:
In [1]:
%run ../linked_list/linked_list.py
In [2]:
class MyLinkedList(LinkedList):
def find_loop_start(self):
if self.head is None or self.head.next is None:
return
i = self.head
j = self.head
i = i.next
j = j.next.next
# Increment i and j until they meet
# j is incremented twice as fast as i
while j != i:
i = i.next
if j is None or j.next is None:
return
j = j.next.next
# When i and j meet, move j to the head
j = self.head
# Increment i and j one node at a time until
# they meet, which is the start of the loop
while j != i:
i = i.next
j = j.next
return i.data
In [3]:
%%writefile test_find_loop_start.py
from nose.tools import assert_equal
class TestFindLoopStart(object):
def test_find_loop_start(self):
print('Test: Empty list')
linked_list = MyLinkedList()
assert_equal(linked_list.find_loop_start(), None)
print('Test: Not a circular linked list: One element')
head = Node(1)
linked_list = MyLinkedList(head)
assert_equal(linked_list.find_loop_start(), None)
print('Test: Not a circular linked list: Two elements')
linked_list.append(2)
assert_equal(linked_list.find_loop_start(), None)
print('Test: Not a circular linked list: Three or more elements')
linked_list.append(3)
assert_equal(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)
assert_equal(linked_list.find_loop_start(), 3)
print('Success: test_find_loop_start')
def main():
test = TestFindLoopStart()
test.test_find_loop_start()
if __name__ == '__main__':
main()
In [4]:
%run -i test_find_loop_start.py