interactive-coding-challenges/linked_lists/delete_mid/delete_mid_solution.ipynb

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

Solution Notebook

Problem: Delete a node in the middle, given only access to that node.

• Constraints
• Test Cases
• Algorithm
• Code
• Unit Test

Constraints

Problem statements are sometimes ambiguous. Identifying constraints and stating assumptions can help to ensure you code the intended solution.

• What if the final node is being deleted, for example a single node list? Do we make it a dummy with value None?
  • Yes
• Can we assume we already have a linked list class that can be used for this problem?
  • Yes

Test Cases

• Delete on empty list -> None
• Delete None -> None
• Delete on one node -> [None]
• Delete on multiple nodes

Algorithm

We'll need two pointers, one to the current node and one to the next node. We will copy the next node's data to the current node's data (effectively deleting the current node) and update the current node's next pointer.

• set curr.data to next.data
• set curr.next to next.next

Complexity:

• Time: O(1)
• Space: O(1)

Code

In [1]:

%run ../linked_list/linked_list.py

In [2]:

class MyLinkedList(LinkedList):
    
    def delete_node(self, node):
        if self.head is None:
            return
        if node is None:
            return
        node_next = node.next
        if node_next is None:
            node.data = None
            node.next = None
        else:
            node.data = node_next.data
            node.next = node_next.next

Unit Test

In [3]:

%%writefile test_delete_mid.py
from nose.tools import assert_equal


class TestDeleteNode(object):
    
    def test_delete_node(self):
        print('Test: Empty list, null node to delete')
        linked_list = MyLinkedList(None)
        linked_list.delete_node(None)
        assert_equal(linked_list.get_all_data(), [])

        print('Test: One node')
        head = Node(2)
        linked_list = MyLinkedList(head)
        linked_list.delete_node(head)
        assert_equal(linked_list.get_all_data(), [None])

        print('Test: Multiple nodes')
        linked_list = MyLinkedList(None)        
        node0 = linked_list.insert_to_front(1)
        node1 = linked_list.insert_to_front(3)
        node2 = linked_list.insert_to_front(4)
        node3 = linked_list.insert_to_front(1)
        linked_list.delete_node(node2)
        assert_equal(linked_list.get_all_data(), [1, 3, 1])
        
        print('Success: test_delete_node')

def main():
    test = TestDeleteNode()
    test.test_delete_node()
    
if __name__ == '__main__':
    main()

Overwriting test_delete_mid.py

In [4]:

%run -i test_delete_mid.py

Test: Empty list, null node to delete
Test: One node
Test: Multiple nodes
Success: test_delete_node