Intro
Last time, we learned how to update/set a specific node.
Today, we learn how to insert a new node at any specific index.
Current Code
We start with code that has push
, unshift
and get
, because we can reuse these methods.
class Node {
constructor(value) {
this.value = value;
this.next = null;
}
}
class SinglyLinkedList {
constructor() {
this.length = 0;
this.head = null;
this.tail = null;
}
push(value) {
const newNode = new Node(value);
if (!this.length) {
this.head = newNode;
} else {
this.tail.next = newNode;
}
this.tail = newNode;
this.length += 1;
return newNode;
}
unshift(value) {
const newHead = new Node(value);
if (!this.length) {
this.head = newHead;
this.tail = newHead;
} else {
newHead.next = this.head;
this.head = newHead;
}
this.length += 1;
return newHead;
}
get(index) {
if (index < 0 || index >= this.length) {
return null;
} else {
let count = 0;
let currentNode = this.head;
while (count < index) {
currentNode = currentNode.next;
count += 1;
}
return currentNode;
}
}
}
Thoughts
First, we should think about the constraints and possibilities:
If we want to add a node "outside" the List (less than 0 or greater than the length of the current List):
- return null
If we want to add a node at the beginning of the List (index is 0):
- we can use our
unshift
method
If we want to add a node at the end of the List:
- we can use our
push
method
All remaining cases:
- create a new node
- put it between the node before the new node's place and the node, that is currently at this place
Example:
- current List: A -> B
- desired List: A -> N -> B
Steps:
- create new node
N
- point
N
'snext
toB
- point
A
'snext
toN
Implementation (Short version, DRY)
class Node {
constructor(value) {
this.value = value;
this.next = null;
}
}
class SinglyLinkedList {
constructor() {
this.length = 0;
this.head = null;
this.tail = null;
}
push(value) {
const newNode = new Node(value);
if (!this.length) {
this.head = newNode;
} else {
this.tail.next = newNode;
}
this.tail = newNode;
this.length += 1;
return newNode;
}
unshift(value) {
const newHead = new Node(value);
if (!this.length) {
this.head = newHead;
this.tail = newHead;
} else {
newHead.next = this.head;
this.head = newHead;
}
this.length += 1;
return newHead;
}
get(index) {
if (index < 0 || index >= this.length) {
return null;
} else {
let count = 0;
let currentNode = this.head;
while (count < index) {
currentNode = currentNode.next;
count += 1;
}
return currentNode;
}
}
insert(index, value) {
// add a node "outside" the List (=> invalid)
if (index < 0 || index > this.length) {
return null;
} else if (index === 0) {
// add a node to the beginning of the List
return this.unshift(value);
} else if (index === this.length) {
// add a node to the end of the List
return this.push(value);
} else {
// get the node before the new node's desired place (because it has to point to the new node soon)
const preDesiredPlace = this.get(index - 1);
// create a new node
const newNode = new Node(value);
// the new node should point to the node, that is currently at the new node's desired place
newNode.next = preDesiredPlace.next;
// the node before the new node's desired place should point to the new node
preDesiredPlace.next = 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 Singly Linked List's insert
method and its results.
const newSLL = new SinglyLinkedList();
console.log(newSLL.insert(0, "A"));
// Node { value: 'A', next: null }
console.log(newSLL.insert(1, "B"));
// Node { value: 'B', next: null }
console.log(newSLL);
// SinglyLinkedList {
// length: 2,
// head: Node { value: 'A', next: Node { value: 'B', next: null } },
// tail: Node { value: 'B', next: null }
// }
console.log(newSLL.insert(1, "N (between A and B)"));
// Node {
// value: 'N (between A and B)',
// next: Node { value: 'B', next: null }
// }
console.log(newSLL);
// SinglyLinkedList {
// length: 3,
// head: Node {
// value: 'A',
// next: Node { value: 'N (between A and B)', next: [Node] }
// },
// tail: Node { value: 'B', next: null }
// }
Next Part
We will implement how to remove a node at a specific index. If you want to be notified, subscribe :)
Questions
We are doing it like:
- point
N
'snext
toB
- point
A
'snext
toN
What would happen, if we'd switch these steps?
Top comments (2)
we can use our pop method -> we can use our push method
Thanks Bruce,
fixed it.