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 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:
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
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
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.