This notebook was prepared by Donne Martin. Source and license info is on GitHub.

Challenge Notebook

Problem: Partition a linked list around a value x, such that all nodes less than x come before all nodes greater than or equal to x.

Constraints

  • Can we assume this is a non-circular, singly linked list?
    • Yes
  • Do we expect the function to return a new list?
    • Yes
  • Can we assume the input x is valid?
    • Yes
  • Can we assume we already have a linked list class that can be used for this problem?
    • Yes
  • Can we create additional data structures?
    • Yes
  • Can we assume this fits in memory?
    • Yes

Test Cases

  • Empty list -> []
  • One element list -> [element]
  • Left linked list is empty
  • Right linked list is empty
  • General case
    • Partition = 10
    • Input: 4, 3, 7, 8, 10, 1, 10, 12
    • Output: 4, 3, 7, 8, 1, 10, 10, 12

Algorithm

Refer to the Solution Notebook. If you are stuck and need a hint, the solution notebook's algorithm discussion might be a good place to start.

Code

In [ ]:
%run ../linked_list/linked_list.py
%load ../linked_list/linked_list.py
In [ ]:
class MyLinkedList(LinkedList):

    def partition(self, data):
        # TODO: Implement me
        pass

Unit Test

The following unit test is expected to fail until you solve the challenge.

In [ ]:
# %load test_partition.py
import unittest


class TestPartition(unittest.TestCase):

    def test_partition(self):
        print('Test: Empty list')
        linked_list = MyLinkedList(None)
        linked_list.partition(10)
        self.assertEqual(linked_list.get_all_data(), [])

        print('Test: One element list, left list empty')
        linked_list = MyLinkedList(Node(5))
        linked_list.partition(0)
        self.assertEqual(linked_list.get_all_data(), [5])

        print('Test: Right list is empty')
        linked_list = MyLinkedList(Node(5))
        linked_list.partition(10)
        self.assertEqual(linked_list.get_all_data(), [5])

        print('Test: General case')
        # Partition = 10
        # Input: 4, 3, 13, 8, 10, 1, 14, 10, 12
        # Output: 4, 3, 8, 1, 10, 10, 13, 14, 12
        linked_list = MyLinkedList(Node(12))
        linked_list.insert_to_front(10)
        linked_list.insert_to_front(14)
        linked_list.insert_to_front(1)
        linked_list.insert_to_front(10)
        linked_list.insert_to_front(8)
        linked_list.insert_to_front(13)
        linked_list.insert_to_front(3)
        linked_list.insert_to_front(4)
        partitioned_list = linked_list.partition(10)
        self.assertEqual(partitioned_list.get_all_data(),
                     [4, 3, 8, 1, 10, 10, 13, 14, 12])

        print('Success: test_partition')


def main():
    test = TestPartition()
    test.test_partition()


if __name__ == '__main__':
    main()

Solution Notebook

Review the Solution Notebook for a discussion on algorithms and code solutions.