Understanding and Implementing the Linked List Pop Method in JavaScript

In the realm of data structures, linked lists play a crucial role, offering a dynamic and flexible way to store and manage data. One essential operation in linked lists is the pop method, which allows us to efficiently remove the last element. In this article, we will explore the intricacies of the pop method, providing a step-by-step guide for its implementation in JavaScript.

1. Overview of Linked Lists:

Linked lists are linear data structures consisting of nodes, where each node holds data and a reference (or link) to the next node in the sequence. Unlike arrays, linked lists allow for dynamic resizing and efficient insertion and deletion.

The pop method in linked lists is particularly useful when we want to remove the last element, adjusting the structure accordingly.

2. Node Class Implementation:

Before diving into the pop method, let’s briefly introduce the Node class, which forms the basic building block of linked lists. The Node class holds a value and a reference to the next node.

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

In simple terms, a Node is like a building block in our linked list. It contains a piece of information (value) and a pointer (next) to the next building block in the list.

3. Linked List Class Initialization:

Now, let’s initialize the Linkedlist class, which contains a head, a tail, and a length property. We’ll also add a push method to easily add elements to the linked list.

class Linkedlist {
    constructor(value) {
        const newNode = new Node(value);
        this.head = newNode;
        this.tail = this.head;
        this.length = 1;
    }

    push(value) {
        const newNode = new Node(value);
        if (!this.head) {
            this.head = newNode;
            this.tail = newNode;
        } else {
            this.tail.next = newNode;
            this.tail = newNode;
        }
        this.length++;
        return this;
    }
    // (Pop method implementation)
}

4. Push Method:

Before we proceed to the pop method, let’s quickly recap the push method. The push method adds a new node to the end of the linked list, updating the tail and length accordingly.

5. Understanding the Pop Method:

The pop method focuses on removing the last element from the linked list. However, there are edge cases to consider: an empty list, a list with one item, and a list with two or more items. We’ll use temporary variables to iterate through the list and efficiently update pointers.

6. Step-by-Step Implementation:

Now, let’s implement the pop method step by step. We’ll address scenarios of zero, one, and multiple items in the list. The key is to use temporary variables, iterate through the list, and update pointers accordingly.

pop() {
    if (!this.head) {
        return undefined; // Empty list
    }

    let temp = this.head;
    let pre = this.head;

    while (temp.next) {
        pre = temp;
        temp = temp.next;
    }

    this.tail = pre;
    this.tail.next = null;
    this.length--;

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

    return temp;
}

Now, let’s break down the pop method:

Empty List Check:

• If the train (linked list) is empty, there’s nothing to remove, so we return undefined.

Setting Up Temporary Variables:

• We have two “pointers,” temp and pre, both initially pointing to the first carriage (head).

Iterating Through the List:

• We loop through the train until we reach the last carriage (tail).
• temp.next checks if there's another carriage after the current one.
• Inside the loop, pre is always one step behind temp.

Updating Pointers:

• Once the loop ends, temp is at the last carriage, and pre is at the one before the last.
• We update the tail to be the carriage before the last, effectively detaching the last carriage.
• We set the tail.next to null to signify the end of the train.

Handling Edge Cases:

• If the length becomes zero after removing the last carriage, we set both head and tail to null.

Returning the Removed Node:

• Finally, we return the last carriage that was removed.

7. Testing the Pop Method:

Let’s test the pop method using a sample linked list. We’ll demonstrate scenarios with two or more items, one item, and zero items, ensuring the correctness of our implementation.

let mylinkedList = new Linkedlist(66);
mylinkedList.push(77);

console.log(mylinkedList.pop()); // Testing pop with two or more items
console.log(mylinkedList.pop()); // Testing pop with one item
console.log(mylinkedList.pop()); // Testing pop with zero items

8. Final Code and Usage:

With the pop method implemented, you now have a versatile linked list in JavaScript. The final code includes the Node and Linkedlist classes along with the push and pop methods. Use this code in your projects to handle dynamic data structures efficiently.

In conclusion, the pop method is a crucial operation in linked lists, enabling the removal of elements with efficiency. This article has provided a comprehensive guide to understanding and implementing the pop method in JavaScript. Whether you’re a novice or an experienced developer, mastering linked list operations is fundamental to building robust applications.

Happy Coding! — Bhagya Wijenayake