JaeHyoLee's solution to Linked List on the Lua track

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.

Running the tests

To run the tests, run the command busted from within the exercise directory.

Further information

For more detailed information about the Lua track, including how to get help if you're having trouble, please visit the exercism.io Lua language page.

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_spec.lua

local LinkedList = require('linked-list')

describe('linked-list', function()
  it('should be able to pop pushed elements', function()
    local list = LinkedList()
    list:push(10)
    list:push(20)
    assert.equal(20, list:pop())
    assert.equal(10, list:pop())
  end)

  it('should be able to shift pushed elements', function()
    local list = LinkedList()
    list:push(10)
    list:push(20)
    assert.equal(10, list:shift())
    assert.equal(20, list:shift())
  end)

  it('should be able to shift unshifted elements', function()
    local list = LinkedList()
    list:unshift(10)
    list:unshift(20)
    assert.equal(20, list:shift())
    assert.equal(10, list:shift())
  end)

  it('should be able to pop unshifted elements', function()
    local list = LinkedList()
    list:unshift(10)
    list:unshift(20)
    assert.equal(10, list:pop())
    assert.equal(20, list:pop())
  end)

  it('should be able to count its elements', function()
    local list = LinkedList()
    assert.equal(0, list:count())
    list:push(10)
    assert.equal(1, list:count())
    list:push(20)
    assert.equal(2, list:count())

  end)

  it('should count correctly after a shift', function()
    local list = LinkedList()
    list:push(10)
    list:push(20)
    list:shift()
    assert.equal(1, list:count())
  end)

  it('should count correctly after a pop', function()
    local list = LinkedList()
    list:push(10)
    list:push(20)
    list:pop()
    assert.equal(1, list:count())
  end)

  it('should be able to delete from the beginning of the list', function()
    local list = LinkedList()
    list:push(10)
    list:push(20)
    list:push(30)
    list:delete(30)
    assert.equal(2, list:count())
    assert.equal(20, list:pop())
    assert.equal(10, list:shift())
  end)

  it('should be able to delete from the middle of the list', function()
    local list = LinkedList()
    list:push(10)
    list:push(20)
    list:push(30)
    list:delete(20)
    assert.equal(2, list:count())
    assert.equal(30, list:pop())
    assert.equal(10, list:shift())
  end)

  it('should be able to delete from the end of the list', function()
    local list = LinkedList()
    list:push(10)
    list:push(20)
    list:push(30)
    list:delete(10)
    assert.equal(2, list:count())
    assert.equal(30, list:pop())
    assert.equal(20, list:shift())
  end)

  it('should delete all elements with the matching value', function()
    local list = LinkedList()
    list:push(10)
    list:push(20)
    list:push(20)
    list:push(30)
    list:delete(20)
    assert.equal(2, list:count())
    assert.equal(30, list:pop())
    assert.equal(10, list:shift())
  end)

  it('should be able to delete the only element', function()
    local list = LinkedList()
    list:push(10)
    list:delete(10)
    assert.equal(0, list:count())
  end)
end)

local LinkedList = {}
LinkedList.__index = LinkedList

function LinkedList:push(n)
  local node = {
    value = n,
    prev = self.list.tail,
    next = nil }
  if self.list.count == 0 then
    self.list.head = node
    self.list.tail = node
  else
    self.list.tail.next = node
    self.list.tail = node
  end
  self.list.count = self.list.count + 1
end

function LinkedList:pop()
  if self.list.count >0 then
    local value = self.list.tail.value
    if self.list.tail.prev ~= nil then
      self.list.tail.prev.next = nil
      self.list.tail = self.list.tail.prev
    end
    self.list.count = self.list.count - 1
    return value
  end
end

function LinkedList:unshift(n)
  local node = {
    value = n,
    prev = nil,
    next = self.list.head }
  if self.list.count == 0 then
    self.list.head = node
    self.list.tail = node
  else
    self.list.head.prev = node
    self.list.head = node
  end
  self.list.count = self.list.count + 1
end

function LinkedList:shift()
  if self.list.count > 0 then
    local value = self.list.head.value
    if self.list.head.next ~= nil then
      self.list.head.next.prev = nil
      self.list.head = self.list.head.next
    end
    self.list.count = self.list.count - 1
    return value
  end
end

function LinkedList:count()
  return self.list.count
end

function LinkedList:delete(n)
  local node = self.list.head
  while node ~= nil do
    if node.value == n then
      if node.prev ~= nil then node.prev.next = node.next
      else self.list.head = node.next end
      if node.next ~= nil then node.next.prev = node.prev
      else self.list.tail = node.prev end
      self.list.count = self.list.count - 1
    end
    node = node.next
  end
end

return function()
  local self = setmetatable({}, LinkedList)
  self.list = {head = nil, tail = nil, count = 0}
  return self
end

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JaeHyoLee's solution

to Linked List in the Lua Track