In this article, we will be implementing the LinkedList data structure in Javascript. LinkedList is the dynamic data structure, as we can add or remove elements at ease, and it can even grow as needed. Just like arrays, linked lists store elements sequentially, but don’t store the elements contiguously like an array.
Example 1: In the below code example, we define a class Node having two properties: element and next. Element holds the data of a node while next holds the pointer to the next node, which is initialized to the null value.
// insert element at the position index // of the list insertAt(element, index) { if (index < 0 || index > this .size)
return console.log( "Please enter a valid index." );
else {
// creates a new node
let node = new Node(element);
let curr, prev;
curr = this .head;
// add the element to the
// first index
if (index == 0) {
node.next = this .head;
this .head = node;
} else {
curr = this .head;
let it = 0;
// iterate over the list to find
// the position to insert
while (it < index) {
it++;
prev = curr;
curr = curr.next;
}
// adding an element
node.next = curr;
prev.next = node;
}
this .size++;
}
} |
Example 2: This example, shows a Linked List class with a constructor and a list of methods to be implemented. Linked List class has two properties: i.e. head and size, where the head stores the first node of a List, and size indicates the number of nodes in a list.
// removes an element from the // specified location removeFrom(index) { if (index < 0 || index >= this .size)
return console.log( "Please Enter a valid index" );
else {
let curr, prev, it = 0;
curr = this .head;
prev = curr;
// deleting first element
if (index === 0) {
this .head = curr.next;
} else {
// iterate over the list to the
// position to remove an element
while (it < index) {
it++;
prev = curr;
curr = curr.next;
}
// remove the element
prev.next = curr.next;
}
this .size--;
// return the remove element
return curr.element;
}
} |
Let’s implement each of these functions:
1. add(element) – It adds an element at the end of the list.
Example: Below is the code example.
// removes a given element from the // list removeElement(element) { let current = this .head;
let prev = null ;
// iterate over the list
while (current != null ) {
// comparing element with current
// element if found then remove the
// and return true
if (current.element === element) {
if (prev == null ) {
this .head = current.next;
} else {
prev.next = current.next;
}
this .size--;
return current.element;
}
prev = current;
current = current.next;
}
return -1;
} |
In the order to add an element at the end of the list we consider the following :
- If the list is empty then add an element and it will be head
- If the list is not empty then iterate to the end of the list and add an element at the end of the list
2. insertAt(element, index) – It inserts an element at the given index in a list.
Example: Below is the code example.
// finds the index of element indexOf(element) { let count = 0;
let current = this .head;
// iterate over the list
while (current != null ) {
// compare each element of the list
// with given element
if (current.element === element)
return count;
count++;
current = current.next;
}
// not found
return -1;
} |
In order to add an element at the given index of the list we consider three conditions as follows:
- if the index is zero we add an element at the front of the list and make it head
- If the index is the last position of the list we append the element at the end of the list
- if the index is between 0 or size – 1 we iterate over to the index and add an element at that index
3. removeFrom(index) – It removes and returns an element from the list from the specified index
Example: Below is the code example.
// checks the list for empty isEmpty() { return this .size == 0;
} |
In order to remove an element from the list we consider three conditions:
- If the index is 0 then we remove the head and make the next node head of the list
- if the index is size – 1 then we remove the last element from the list and make prev the last element
- if it’s in between 0 to size – 1 we remove the element by using prev and the current node
4. removeElement(element) – This method removes element from the list. It returns the removed element, or if it’s not found it returns -1.
Example: Below is the code example.
// gives the size of the list size_of_list() { console.log( this .size);
} |
The above method is just a modification of removeFrom(index), as it searches for an element and removes it, rather than removing it from a specified location
Helper Methods
Let’s declare some helper methods which are useful while working with LinkedList.
1. indexOf(element) – it returns the index of a given element if the element is in the list.
Example: Below is the code example.
// prints the list items printList() { let curr = this .head;
let str = "" ;
while (curr) {
str += curr.element + " " ;
curr = curr.next;
}
console.log(str);
} |
In this method, we iterate over the list to find the index of an element. If it is not present in the list it returns -1 instead.
2. isEmpty() – it returns true if the list is empty.
Example: Below is the code example.
// creating an object for the // Linkedlist class let ll = new LinkedList();
// testing isEmpty on an empty list // returns true console.log(ll.isEmpty()); // adding element to the list ll.add(10); // prints 10 ll.printList(); // returns 1 console.log(ll.size_of_list()); // adding more elements to the list ll.add(20); ll.add(30); ll.add(40); ll.add(50); // returns 10 20 30 40 50 ll.printList(); // prints 50 from the list console.log( "is element removed ?" + ll.removeElement(50));
// prints 10 20 30 40 ll.printList(); // returns 3 console.log( "Index of 40 " + ll.indexOf(40));
// insert 60 at second position // ll contains 10 20 60 30 40 ll.insertAt(60, 2); ll.printList(); // returns false console.log( "is List Empty ? " + ll.isEmpty());
// remove 3rd element from the list console.log(ll.removeFrom(3)); // prints 10 20 60 40 ll.printList(); |
In this method we check for the size property of the LinkedList class, and if it’s zero then the list is empty.
3. size_of_list() – It returns the size of list
Example: Below is the code example.
class Node { // constructor
constructor(element) {
this .element = element;
this .next = null
}
} // linkedlist class class LinkedList { constructor() {
this .head = null ;
this .size = 0;
}
// adds an element at the end
// of list
add(element) {
// creates a new node
let node = new Node(element);
// to store current node
let current;
// if list is Empty add the
// element and make it head
if ( this .head == null )
this .head = node;
else {
current = this .head;
// iterate to the end of the
// list
while (current.next) {
current = current.next;
}
// add node
current.next = node;
}
this .size++;
}
// insert element at the position index
// of the list
insertAt(element, index) {
if (index < 0 || index > this .size)
return console.log( "Please enter a valid index." );
else {
// creates a new node
let node = new Node(element);
let curr, prev;
curr = this .head;
// add the element to the
// first index
if (index == 0) {
node.next = this .head;
this .head = node;
} else {
curr = this .head;
let it = 0;
// iterate over the list to find
// the position to insert
while (it < index) {
it++;
prev = curr;
curr = curr.next;
}
// adding an element
node.next = curr;
prev.next = node;
}
this .size++;
}
}
// removes an element from the
// specified location
removeFrom(index) {
if (index < 0 || index >= this .size)
return console.log( "Please Enter a valid index" );
else {
let curr, prev, it = 0;
curr = this .head;
prev = curr;
// deleting first element
if (index === 0) {
this .head = curr.next;
} else {
// iterate over the list to the
// position to remove an element
while (it < index) {
it++;
prev = curr;
curr = curr.next;
}
// remove the element
prev.next = curr.next;
}
this .size--;
// return the remove element
return curr.element;
}
}
// removes a given element from the
// list
removeElement(element) {
let current = this .head;
let prev = null ;
// iterate over the list
while (current != null ) {
// comparing element with current
// element if found then remove the
// and return true
if (current.element === element) {
if (prev == null ) {
this .head = current.next;
} else {
prev.next = current.next;
}
this .size--;
return current.element;
}
prev = current;
current = current.next;
}
return -1;
}
// finds the index of element
indexOf(element) {
let count = 0;
let current = this .head;
// iterate over the list
while (current != null ) {
// compare each element of the list
// with given element
if (current.element === element)
return count;
count++;
current = current.next;
}
// not found
return -1;
}
// checks the list for empty
isEmpty() {
return this .size == 0;
}
// gives the size of the list
size_of_list() {
console.log( this .size);
}
// prints the list items
printList() {
let curr = this .head;
let str = "" ;
while (curr) {
str += curr.element + " " ;
curr = curr.next;
}
console.log(str);
}
} // creating an object for the // Linkedlist class let ll = new LinkedList();
// testing isEmpty on an empty list // returns true console.log(ll.isEmpty()); // adding element to the list ll.add(10); // prints 10 ll.printList(); // returns 1 console.log(ll.size_of_list()); // adding more elements to the list ll.add(20); ll.add(30); ll.add(40); ll.add(50); // returns 10 20 30 40 50 ll.printList(); // prints 50 from the list console.log( "is element removed ?" + ll.removeElement(50));
// prints 10 20 30 40 ll.printList(); // returns 3 console.log( "Index of 40 " + ll.indexOf(40));
// insert 60 at second position // ll contains 10 20 60 30 40 ll.insertAt(60, 2); ll.printList(); // returns false console.log( "is List Empty ? " + ll.isEmpty());
// remove 3rd element from the list console.log(ll.removeFrom(3)); // prints 10 20 60 40 ll.printList(); |
3. printList() – It prints the contents of the list.
Example: Below is the code example.
In this method, we iterate over the entire list and concatenate the elements of each node and print it.
Note: Different helper methods can be declared in the LinkedList class as required.
Implementation
Now, let’s use the LinkedList class and its different methods described above.
Example: Below is the code example.
// creating an object for the // Linkedlist class let ll = new LinkedList();
// testing isEmpty on an empty list // returns true console.log(ll.isEmpty()); // adding element to the list ll.add(10); // prints 10 ll.printList(); // returns 1 console.log(ll.size_of_list()); // adding more elements to the list ll.add(20); ll.add(30); ll.add(40); ll.add(50); // returns 10 20 30 40 50 ll.printList(); // prints 50 from the list console.log( "is element removed ?" + ll.removeElement(50));
// prints 10 20 30 40 ll.printList(); // returns 3 console.log( "Index of 40 " + ll.indexOf(40));
// insert 60 at second position // ll contains 10 20 60 30 40 ll.insertAt(60, 2); ll.printList(); // returns false console.log( "is List Empty ? " + ll.isEmpty());
// remove 3rd element from the list console.log(ll.removeFrom(3)); // prints 10 20 60 40 ll.printList(); |
Example: Below is the Complete Code.
class Node { // constructor
constructor(element) {
this .element = element;
this .next = null
}
} // linkedlist class class LinkedList { constructor() {
this .head = null ;
this .size = 0;
}
// adds an element at the end
// of list
add(element) {
// creates a new node
let node = new Node(element);
// to store current node
let current;
// if list is Empty add the
// element and make it head
if ( this .head == null )
this .head = node;
else {
current = this .head;
// iterate to the end of the
// list
while (current.next) {
current = current.next;
}
// add node
current.next = node;
}
this .size++;
}
// insert element at the position index
// of the list
insertAt(element, index) {
if (index < 0 || index > this .size)
return console.log( "Please enter a valid index." );
else {
// creates a new node
let node = new Node(element);
let curr, prev;
curr = this .head;
// add the element to the
// first index
if (index == 0) {
node.next = this .head;
this .head = node;
} else {
curr = this .head;
let it = 0;
// iterate over the list to find
// the position to insert
while (it < index) {
it++;
prev = curr;
curr = curr.next;
}
// adding an element
node.next = curr;
prev.next = node;
}
this .size++;
}
}
// removes an element from the
// specified location
removeFrom(index) {
if (index < 0 || index >= this .size)
return console.log( "Please Enter a valid index" );
else {
let curr, prev, it = 0;
curr = this .head;
prev = curr;
// deleting first element
if (index === 0) {
this .head = curr.next;
} else {
// iterate over the list to the
// position to remove an element
while (it < index) {
it++;
prev = curr;
curr = curr.next;
}
// remove the element
prev.next = curr.next;
}
this .size--;
// return the remove element
return curr.element;
}
}
// removes a given element from the
// list
removeElement(element) {
let current = this .head;
let prev = null ;
// iterate over the list
while (current != null ) {
// comparing element with current
// element if found then remove the
// and return true
if (current.element === element) {
if (prev == null ) {
this .head = current.next;
} else {
prev.next = current.next;
}
this .size--;
return current.element;
}
prev = current;
current = current.next;
}
return -1;
}
// finds the index of element
indexOf(element) {
let count = 0;
let current = this .head;
// iterate over the list
while (current != null ) {
// compare each element of the list
// with given element
if (current.element === element)
return count;
count++;
current = current.next;
}
// not found
return -1;
}
// checks the list for empty
isEmpty() {
return this .size == 0;
}
// gives the size of the list
size_of_list() {
console.log( this .size);
}
// prints the list items
printList() {
let curr = this .head;
let str = "" ;
while (curr) {
str += curr.element + " " ;
curr = curr.next;
}
console.log(str);
}
} // creating an object for the // Linkedlist class let ll = new LinkedList();
// testing isEmpty on an empty list // returns true console.log(ll.isEmpty()); // adding element to the list ll.add(10); // prints 10 ll.printList(); // returns 1 console.log(ll.size_of_list()); // adding more elements to the list ll.add(20); ll.add(30); ll.add(40); ll.add(50); // returns 10 20 30 40 50 ll.printList(); // prints 50 from the list console.log( "is element removed ?" + ll.removeElement(50));
// prints 10 20 30 40 ll.printList(); // returns 3 console.log( "Index of 40 " + ll.indexOf(40));
// insert 60 at second position // ll contains 10 20 60 30 40 ll.insertAt(60, 2); ll.printList(); // returns false console.log( "is List Empty ? " + ll.isEmpty());
// remove 3rd element from the list console.log(ll.removeFrom(3)); // prints 10 20 60 40 ll.printList(); |
true 10 1 undefined 10 20 30 40 50 is element removed ?50 10 20 30 40 Index of 40 3 10 20 60 30 40 is List Empty ? false 30 10 20 60 40
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