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
shift will never be called on an
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:
To include color from the command line:
ruby -r minitest/pride linked_list_test.rb
Classic computer science topic
It's possible to submit an incomplete solution so you can see how others have completed the exercise.
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
A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.
Here are some questions to help you reflect on this solution and learn the most from it.