# JavaScript Data Structures: Doubly Linked List: Insert a new node at a specific index

## Intro

Last time, we learned how to update a specific node.

Today, we'll learn how to insert a new node at a specific index.

## Starter Code

We start with code that has the `push`, `unshift` and `get` method,
because we can reuse them to add data.

``````class Node {
constructor(value) {
this.value = value;
this.prev = null;
this.next = null;
}
}

constructor() {
this.length = 0;
this.tail = null;
}

push(value) {
const newNode = new Node(value);
if (!this.length) {
this.tail = newNode;
} else {
this.tail.next = newNode;
newNode.prev = this.tail;
this.tail = newNode;
}
this.length += 1;
return newNode;
}

unshift(value) {
const newNode = new Node(value);

if (!this.length) {
this.tail = newNode;
} else {
}

this.length += 1;
return newNode;
}

get(index) {
if (!this.length || index < 0 || index >= this.length) {
return null;
} else {
let currentNode;

if (index < this.length / 2) {
let counter = 0;

while (counter < index) {
currentNode = currentNode.next;
counter += 1;
}
} else {
let counter = this.length - 1;

currentNode = this.tail;

while (counter > index) {
currentNode = currentNode.prev;
counter -= 1;
}
}

return currentNode;
}
}
}
``````

## Thoughts

First, we should think about the constraints and possibilities:

If the index is less than 0:

• return null

If the index is greater than the list's length:

• return null

If the index equals 0:

• use the `unshift` method to add the data

If the index equals length:

• use the `push` method to add the data

All remaining cases:

• create a new node
• find the node that is currently before the desired place and connect it to the new node
• find the node that is currently at the desired place and connect it to the new node
• increase the list's length by 1
• return the new node

## Example

``````// current list:
A <===> B
// desired list:
A <===> X <===> B
``````

Steps:

``````// current list:
A <===> B

// find the node that is currently before the desired place and connect it to the new node
// there is still the connection from B.prev to A
A <===> X
A <==   B

// find the node that is currently at the desired place and connect it to the new node
A <===> X <===> B

// desired list:
A <===> X <===> B
``````

=> list after last step equals the desired list

## Implementation (Short)

``````class Node {
constructor(value) {
this.value = value;
this.prev = null;
this.next = null;
}
}

constructor() {
this.length = 0;
this.tail = null;
}

push(value) {
const newNode = new Node(value);
if (!this.length) {
this.tail = newNode;
} else {
this.tail.next = newNode;
newNode.prev = this.tail;
this.tail = newNode;
}
this.length += 1;
return newNode;
}

unshift(value) {
const newNode = new Node(value);

if (!this.length) {
this.tail = newNode;
} else {
}

this.length += 1;
return newNode;
}

get(index) {
if (!this.length || index < 0 || index >= this.length) {
return null;
} else {
let currentNode;

if (index < this.length / 2) {
let counter = 0;

while (counter < index) {
currentNode = currentNode.next;
counter += 1;
}
} else {
let counter = this.length - 1;

currentNode = this.tail;

while (counter > index) {
currentNode = currentNode.prev;
counter -= 1;
}
}

return currentNode;
}
}

insert(index, value) {
// if the index is less than 0 or greater than the list's length, return null
if (index < 0 || index > this.length) {
return null;
} else if (index === 0) {
// if the index equals 0, use the `unshift` method
return this.unshift(value);
} else if (index === this.length) {
// if the index equals length, use the `push` method
return this.push(value);
} else {
// create new node
const newNode = new Node(value);

// find the new previous node
const newPrevNode = this.get(index - 1);
// find the new next node
const newNextNode = newPrevNode.next;

// connect the new node to the new previous node
newNode.prev = newPrevNode;
newPrevNode.next = newNode;

// connect the new node to the new next node
newNode.next = newNextNode;
newNextNode.prev = newNode;

// increase the list's length by 1
this.length += 1;

// return the new node
return newNode;
}
}
}
``````

## Result

Let's have a look how to use the Doubly Linked List's `insert` method and its results.

``````const newDLL = new DoublyLinkedList();

// index too low
console.log(newDLL.insert(-1, "too low"));
// null

// shoould display the new node
console.log(newDLL.insert(0, "at 0"));
// Node { value: 'at 0', prev: null, next: null }

// shoould display the new node
console.log(newDLL.insert(1, "at 1"));
// <ref *1> Node {
//   value: 'at 1',
//   prev: Node { value: 'at 0', prev: null, next: [Circular *1] },
//   next: null
// }

// should insert the node between the other two nodes
console.log(newDLL.insert(1, "new at 1"));
// <ref *1> Node {
//   value: 'new at 1',
//   prev: Node { value: 'at 0', prev: null, next: [Circular *1] },
//   next: Node { value: 'at 1', prev: [Circular *1], next: null }
// }

// should show three nodes in the list: at 0 => new at 1 => at 1
console.log(newDLL);
//   length: 3,
//   head: <ref *1> Node {
//     value: 'at 0',
//     prev: null,
//     next: Node { value: 'new at 1', prev: [Circular *1], next: [Node] }
//   },
//   tail: <ref *2> Node {
//     value: 'at 1',
//     prev: Node { value: 'new at 1', prev: [Node], next: [Circular *2] },
//     next: null
//   }
// }
``````

## Next Part

We will implement our next method for the Doubly Linked List: `remove` a specific node.

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### Discussion   