Doubly Linked List

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Doubly Linked List

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Create a linked list that allows easy traversal in both forward and back directions.

Hint: The nodes will look like this:

* Linked list node
export type DoublyLinkedListNode<T> = {
prev: DoublyLinkedListNode<T> | null;
value: T;
next: DoublyLinkedListNode<T> | null;


* Linked list node
export type DoublyLinkedListNode<T> = {
prev: DoublyLinkedListNode<T> | null;
value: T;
next: DoublyLinkedListNode<T> | null;

* Linked list for items of type T
export class DoublyLinkedList<T> {
head: DoublyLinkedListNode<T> | null = null;
tail: DoublyLinkedListNode<T> | null = null;

* Adds an item in O(1)
add(value: T) {
const node: DoublyLinkedListNode<T> = {
prev: null,
next: null,
if (!this.head) {
this.head = node;
if (this.tail) { = node;
node.prev = this.tail;
this.tail = node;

* FIFO removal in O(1)
dequeue(): T | null {
if (!this.head) return null;
const value = this.head.value;
this.head =;
if (!this.head) {
this.tail = null;
} else {
this.head.prev = null;
return value;

* LIFO removal in O(1)
pop(): T | null {
if (!this.tail) return null;
const value = this.tail.value;
this.tail = this.tail.prev;
if (!this.tail) {
this.head = null;
} else { = null;
return value;

* Returns an iterator over the values
*values() {
let current = this.head;
while (current) {
yield current.value;
current =;


00:00 In our last lesson we looked at the link list data

00:02 structure and its key value proposition,

00:04 which is its super fast DQ operation.

00:07 In this lesson, we will look at a key limitation

00:09 of the link list data structure along with

00:11 how we can solve it by going w linked.

00:14 So let's go. Here's the quick recap.

00:17 Shown on screen is the DQ operation

00:20 of the link list TETA structure.

00:22 We can easily do a first in first out

00:24 removal in constant time.

00:26 This is because we can get the first added value using head

00:29 dot value and then

00:30 because we are going to be D Qing it,

00:32 we will update the head to the next using head do next.

00:36 Beyond that, there's a simple slight maintenance

00:39 of the invariant that when we run out of items

00:41 that is head becomes null,

00:43 then tail should also be set to null.

00:45 Similar to the DQ operation, which is first

00:48 and first out, we could also consider adding a last

00:51 and first out operation,

00:52 which is also commonly called pop in.

00:54 The body of the function will be very similar.

00:57 We have access to the tail of the link list

00:59 so we can grab the last added value from this to tail value.

01:03 And now after this value will be removed, we will need

01:06 to update the tail to be the second last value

01:08 that was added, which we should be able to grab from tail

01:12 or previous, but we are not storing previous

01:14 because this is a single link list.

01:17 Here's a quick comment explaining

01:19 what our current node structure looks like.

01:21 Each node has a value

01:23 and a next member pointing to the next node in the chain.

01:26 This means we can easily go in the forward direction

01:29 with node next, next,

01:30 but there is no way to quickly turn back with node previous.

01:35 Now hopefully the solution to the problem is pretty obvious.

01:37 In addition to storing next references like the single link

01:40 list, the W link list also stores reference

01:43 to the previous element in a dot prev member.

01:46 And with this people have the W link list data structure,

01:48 which is capable of doing first in first out removal in OF

01:52 one using DQ as well as last

01:54 and first of removal in OF one using pop.

01:57 Now you might recall that in the link list data structure,

02:00 there are essentially two things, the link list node

02:02 and the link list class.

02:04 The first thing that we do is to modify the link list node

02:07 to be the double link list node.

02:09 And then just like the next member,

02:11 we will also be persisting the prev member.

02:13 And beyond the name difference, the data type for the next

02:16 and the prev members is exactly the same.

02:18 Next we update the link, this class

02:20 to be called the W link list.

02:23 Now let's start modifying the methods of the W link list so

02:25 that the next and the prev members are correctly maintained.

02:29 First up, the ad method.

02:31 When we create a new node, we will have

02:33 to specify a previous member as well,

02:34 just like we specify the next member

02:36 and we will set it to now.

02:38 And if our link list already has a tail,

02:41 then this new node is going

02:42 to have its previous member pointing to that tail,

02:45 and that's the extent of the changes we

02:47 need for the add method.

02:49 Next, let's jump into the DQ method.

02:52 And again, the changes over here are going to be minimal

02:55 When we remove the head, we have a new head, but this head

02:58 Is going to point to the head that we just removed.

03:01 That is the head of prev is going to point to that,

03:03 so we have to set that to now, and that's it.

03:07 The original methods are now modified

03:09 and we can continue our journey on the method

03:11 that we actually wanted to add, which was pop.

03:13 Now that we have the next and the prev members, the error

03:16 that we had previously on this ate prev has gone away.

03:20 And in terms of function implementation, the DQ

03:22 and the POP methods are very similar.

03:24 We could essentially copy the DQ method

03:26 and paste it into pop

03:27 and replace all instances of head with tail of tail,

03:31 with head of next, with prev and of prev with next,

03:34 and we'd be done, but the code is pretty simple anyways.

03:38 If you don't have a tail, we immediately return now,

03:41 then we grab the value that we have

03:43 to return from tail value

03:45 and this is what we will eventually return.

03:48 Beyond that, there's a slight data structure maintenance.

03:50 We update the tail to be the second last item,

03:53 and if this means that we don't have a tail anymore,

03:55 then we should also set head to Now.

03:57 Finally, if we do have a new tail,

03:59 then we should set it next

04:01 to now essentially popping the old tail

04:04 and that's it for the pop operation.

04:06 Let's jump to the bottom and do a quick demo.

04:09 We create a new WD Ling list, add some items to it,

04:12 which are 1, 3, 6, 9, and 12 in that order.

04:15 And now if we dq the first item, we should get one,

04:18 and if we pop the last item, we should get 12.

04:21 And the remaining items in the link list should be 3, 6, 9.

04:25 And of course, if we execute it,

04:27 it'll work exactly as expected.

04:31 Now, just like we mentioned in the link list lesson,

04:33 you don't actually have to memorize this code

04:35 and you can instead focus on maintaining

04:38 a F key In variance.

04:39 The head and the tail need to be the first

04:41 and the last nodes,

04:42 and they both need to be set

04:43 to null when there are no nodes, the next

04:46 and the previous members need to point to vet elements,

04:49 and with that you should be able to code it up

04:51 during a coding interview.