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to Linked List in the Ruby Track

Published at Jul 13 2018 · 0 comments
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Test suite
Solution

Note:

This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

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 exercism help 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
# Deque
class Deque
  # Element
  Element = Struct.new(:data, :next, :prev) do
  end

  attr_reader :head, :tail
  def initialize
    reset!
  end

  def push(data)
    new_element = Element.new(data)
    return @head = new_element && @tail = new_element if empty?
    new_element.prev = tail
    tail.next = new_element
    @tail = new_element
  end

  def unshift(data)
    new_element = Element.new(data)
    return @head = new_element && @tail = new_element if empty?
    new_element.next = head
    head.prev = new_element
    @head = new_element
  end

  def pop
    prev_tail = tail
    if one_element?
      reset!
    else
      @tail = prev_tail.prev
      prev_tail.next = false
    end
    prev_tail.data
  end

  def shift
    prev_head = head
    if one_element?
      reset!
    else
      @head = prev_head.next
      prev_head.prev = false
    end
    prev_head.data
  end

  private

  def reset!
    @head = false
    @tail = false
  end

  def one_element?
    head == tail
  end

  def empty?
    !head && !tail
  end
end

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