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.
To run the tests, run the command busted
from within the exercise directory.
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.
Classic computer science topic
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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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