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to Linked List in the Ruby Track

Published at May 21 2019 · 0 comments
Instructions
Test suite
Solution

Implement a doubly linked list.

Like an array, a linked list is a simple linear data structure. Several common data types can be implemented using linked lists, like queues, stacks, and associative arrays.

A linked list is a collection of data elements called nodes. In a singly linked list each node holds a value and a link to the next node. In a doubly linked list each node also holds a link to the previous node.

You will write an implementation of a doubly linked list. Implement a Node to hold a value and pointers to the next and previous nodes. Then implement a List which holds references to the first and last node and offers an array-like interface for adding and removing items:

  • push (insert value at back);
  • pop (remove value at back);
  • shift (remove value at front).
  • unshift (insert value at front);

To keep your implementation simple, the tests will not cover error conditions. Specifically: pop or shift will never be called on an empty list.

If you want to know more about linked lists, check Wikipedia.


For installation and learning resources, refer to the Ruby resources page.

For running the tests provided, you will need the Minitest gem. Open a terminal window and run the following command to install minitest:

gem install minitest

If you would like color output, you can require 'minitest/pride' in the test file, or note the alternative instruction, below, for running the test file.

Run the tests from the exercise directory using the following command:

ruby linked_list_test.rb

To include color from the command line:

ruby -r minitest/pride linked_list_test.rb

Source

Classic computer science topic

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

linked_list_test.rb

require 'minitest/autorun'
require_relative 'linked_list'

class DequeTest < Minitest::Test
  def test_push_pop
    deque = Deque.new
    deque.push(10)
    deque.push(20)
    assert_equal 20, deque.pop
    assert_equal 10, deque.pop
  end

  def test_push_shift
    skip
    deque = Deque.new
    deque.push(10)
    deque.push(20)
    assert_equal 10, deque.shift
    assert_equal 20, deque.shift
  end

  def test_unshift_shift
    skip
    deque = Deque.new
    deque.unshift(10)
    deque.unshift(20)
    assert_equal 20, deque.shift
    assert_equal 10, deque.shift
  end

  def test_unshift_pop
    skip
    deque = Deque.new
    deque.unshift(10)
    deque.unshift(20)
    assert_equal 10, deque.pop
    assert_equal 20, deque.pop
  end

  def test_example
    skip
    deque = Deque.new
    deque.push(10)
    deque.push(20)
    assert_equal 20, deque.pop
    deque.push(30)
    assert_equal 10, deque.shift
    deque.unshift(40)
    deque.push(50)
    assert_equal 40, deque.shift
    assert_equal 50, deque.pop
    assert_equal 30, deque.shift
  end

  def test_pop_to_empty
    deque = Deque.new
    deque.push(10)
    assert_equal 10, deque.pop
    deque.push(20)
    assert_equal 20, deque.shift
  end

  def test_shift_to_empty
    deque = Deque.new
    deque.unshift(10)
    assert_equal 10, deque.shift
    deque.unshift(20)
    assert_equal 20, deque.pop
  end
end
class Deque
  class Node
    attr_reader :value
    attr_accessor :prev_node, :next_node

    def initialize(value, prev_node: self, next_node: self)
      @value = value
      @prev_node = prev_node
      @next_node = next_node
    end
  end
  private_constant :Node

  def initialize
    @head = nil
  end

  def push(value)
    if head
      tail = head.prev_node
      new_tail = Node.new(value, prev_node: tail, next_node: head)
      tail.next_node = new_tail
      head.prev_node = new_tail
    else
      self.head = Node.new(value)
    end
  end

  def pop
    if (tail = head.prev_node) == head
      self.head = nil
    else
      head.prev_node = tail.prev_node
      tail.prev_node.next_node = head
    end
    tail.value
  end

  def shift
    if (shifted_head = head) == head.next_node
      self.head = nil
    else
      self.head = shifted_head.next_node
      head.prev_node = shifted_head.prev_node
      head.prev_node.next_node = head
    end
    shifted_head.value
  end

  def unshift(value)
    if head
      new_head = Node.new(value, prev_node: head.prev_node, next_node: head)
      head.prev_node = new_head
      self.head = new_head
    else
      self.head = Node.new(value)
    end
  end

  private

  attr_accessor :head
end

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