JavaScript Exercises: Insert a new node at the beginning of a Singly Linked List

JavaScript Singly Linked List: Exercise-5 with Solution

Write a JavaScript program to insert a node at the beginning of a Singly Linked List.

Sample Solution:

JavaScript Code:

// Define a class representing a node in a singly linked list
class Node {
    constructor(data) {
        // Initialize the node with provided data
        this.data = data
        // Initialize the next pointer to null
        this.next = null                
    }
}

// Define a class representing a singly linked list
class SinglyLinkedList {
    constructor(Head = null) {
        // Initialize the head of the list
        this.Head = Head
    }

    // Method to add a new node to the end of the list
    add(newNode){
        // Start traversal from the head node
        let node = this.Head;
        // If the list is empty, set the new node as the head and return
        if(node == null){
            this.Head = newNode;
            return;
        }
        // Traverse the list until the last node
        while (node.next) {
            node = node.next;
        }
        // Set the next pointer of the last node to the new node
        node.next = newNode;
    }

    // Method to insert a new node at a specified index in the list
    insertAt(index, newNode){
        // Start traversal from the head node
        let node = this.Head;
        // If the specified index is 0, insert the new node at the beginning of the list
        if(index == 0) {
            newNode.next = node;
            this.Head = newNode;
            return;
        } 
        // Traverse the list until the node just before the specified index
        while(--index){
            // If the next node exists, move to the next node
            if(node.next !== null)
                node = node.next;
            // If the next node does not exist and the index is not reached, throw an error
            else
                throw Error("Index Out of Bound");
        }
        // Store the next node of the current node in a temporary variable
        let tempVal = node.next;
        // Set the next pointer of the current node to the new node
        node.next = newNode;
        // Set the next pointer of the new node to the previously stored next node
        newNode.next = tempVal;
    }  

    // Method to insert a new node at the beginning of the list
    insertFirst(value) {   
        // Call the insertAt method with index 0 to insert the node at the beginning
        this.insertAt(0, value);
    }  

    // Method to display the elements of the list
    displayList(){
        // Start traversal from the head node
        let node = this.Head;
        // Initialize an empty string to store the elements of the list
        var str = ""
        // Traverse the list and concatenate each element to the string
        while (node) {
            str += node.data + "->";
            node = node.next;
        }
        // Append "NULL" to indicate the end of the list
        str += "NULL"
        // Print the string containing the list elements
        console.log(str);  
    }
}

// Create an instance of the SinglyLinkedList class
let numList = new SinglyLinkedList();
// Add nodes with data values to the list
numList.add(new Node(12));
numList.add(new Node(13));
numList.add(new Node(14));
numList.add(new Node(15));
// Display the original elements of the list
console.log("Original Linked list:");
numList.displayList();
// Insert a new node with data value 100 at the beginning of the list
console.log("Insert 100 at first position:");
numList.insertFirst(new Node(100));
numList.displayList();
// Insert a new node with data value 10 at the beginning of the list
console.log("Insert 10 at first position:");
numList.insertFirst(new Node(10));
numList.displayList();

Output:

12->13->14->15->NULL
Insert 100 at first position:
100->12->13->14->15->NULL
Insert 10 at first position:
10->100->12->13->14->15->NULL

Flowchart:

Live Demo:

See the Pen javascript-singly-linked-list-exercise-5 by w3resource (@w3resource) on CodePen.

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