This notebook was prepared by [Donne Martin](http://donnemartin.com). Source and license info is on [GitHub](https://github.com/donnemartin/interactive-coding-challenges).

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
Test Cases
Algorithm
Code
Unit Test
Solution Notebook

Constraints

Can we create additional data structures?
- Yes
Do you 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

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
from nose.tools import assert_equal


class TestPartition(object):

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

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

        print('Test: Right list is empty')
        linked_list = MyLinkedList(Node(5))
        linked_list.partition(10)
        assert_equal(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)
        assert_equal(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.