JavaScript Exercises: Head and Tail of a Doubly Linked Lists

JavaScript Data Structures: Exercise-4 with Solution

Get the head and tail of a DLL

Write a JavaScript program to get the head and tail of a Doubly Linked Lists.

Sample Solution:

JavaScript Code:

// Define a class for creating nodes of a doubly linked list
class Node {
	constructor(value) {
		this.value = value; // Store the value of the node
		this.next = null; // Pointer to the next node in the list, initially set to null
		this.previous = null; // Pointer to the previous node in the list, initially set to null
	}
}

// Define a class for creating a doubly linked list
class DoublyLinkedList {
	constructor(value) {
		// Initialize the head node with the given value and no next or previous node
		this.head = {
			value: value, // Store the value of the head node
			next: null, // Pointer to the next node in the list, initially set to null
			previous: null // Pointer to the previous node in the list, initially set to null
		};
		this.length = 0; // Initialize the length of the list to 0
		this.tail = this.head; // Set the tail node to the head node initially
	}

	// Method to add a new node at the end of the list
	add(newNode) {
		// Check if the head is null, indicating an empty list
		if (this.head === null) {
			this.head = newNode; // Set the head to the new node
			this.tail = newNode; // Set the tail to the new node
		} else {
			newNode.previous = this.tail; // Set the previous pointer of the new node to the current tail
			this.tail.next = newNode; // Set the next pointer of the current tail to the new node
			this.tail = newNode; // Update the tail to the new node
		}
		this.length++; // Increment the length of the list
	}

	// Method to get the head node of the list
	get_Head(){
		return this.head; // Return the head node
	}  

	// Method to get the tail node of the list
	get_Tail(){
		return this.tail; // Return the tail node
	}

	// Method to print the values of the nodes in the list
	printList() {
		let current = this.head; // Start from the head of the list
		let result = []; // Array to store the values of the nodes
		while (current !== null) { // Iterate through the list until reaching the end
			result.push(current.value); // Push the value of the current node to the array
			current = current.next; // Move to the next node
		}
		console.log(result.join(' ')); // Log the values of the nodes separated by space
		return this; // Return the DoublyLinkedList object for chaining
	}
}

// Create a new instance of the DoublyLinkedList class
let numList = new DoublyLinkedList();
// Add nodes to the list
numList.add(new Node(2));
numList.add(new Node(3));
numList.add(new Node(5));
numList.add(new Node(6));
numList.add(new Node(8));
// Display the original doubly linked list
console.log("Original Doubly Linked Lists:")
numList.printList();
// Get and display the head node of the list
console.log("Head");
let ghead = numList.get_Head();
console.log(ghead);
// Get and display the tail node of the list
console.log("Tail");
ghead = numList.get_Tail();
console.log(ghead);
// Create a new doubly linked list
let new_numList = new DoublyLinkedList();
console.log("Original Doubly Linked Lists:")
new_numList.printList();
// Get and display the head node of the new list
console.log("Head");
ghead = new_numList.get_Head();
console.log(ghead);
// Get and display the tail node of the new list
console.log("Tail");
ghead = new_numList.get_Tail();
console.log(ghead);

Output:

 "Original Doubly Linked Lists:"
" 2 3 5 6 8"
"Head"
[object Object] {
  next: [object Object] {
    next: [object Object] { ... },
    previous: [circular object Object],
    value: 2
  },
  previous: null,
  value: undefined
}
"Tail"
[object Object] {
  next: null,
  previous: [object Object] {
    next: [circular object Object],
    previous: [object Object] { ... },
    value: 6
  },
  value: 8
}
"Original Doubly Linked Lists:"
""
"Head"
[object Object] {
  next: null,
  previous: null,
  value: undefined
}
"Tail"
[object Object] {
  next: null,
  previous: null,
  value: undefined
} 

Flowchart:

Live Demo:

See the Pen javascript-doubly-linked-list-exercise-4 by w3resource (@w3resource) on CodePen.

For more Practice: Solve these Related Problems:

• Write a JavaScript function that retrieves the head and tail nodes of a doubly linked list and returns them as an object.
• Write a JavaScript function that logs the data of both the head and tail of a DLL after constructing it.
• Write a JavaScript function that traverses a DLL to confirm that the head's previous pointer is null and the tail's next pointer is null.
• Write a JavaScript function that creates a DLL and prints the values at the head and tail to verify correct pointer assignments.

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