Why Pop Using Linked List Node in DSA Javascript?

The Big Idea

What if you could remove the last item instantly without searching through everything?

The Scenario

Imagine you have a stack of books on your desk. You want to remove the top book, but you have to look through all the books one by one to find it because they are not organized well.

The Problem

Manually searching through the books is slow and tiring. You might accidentally remove the wrong book or drop some. It's easy to make mistakes and waste time.

The Solution

Using a linked list with a pop operation lets you quickly remove the top item without searching. It keeps track of the top book, so you just take it off easily and safely.

Before vs After

✗ Before

let books = ['book1', 'book2', 'book3'];
// To remove top book, find last element manually
let topBook = books[books.length - 1];
books.length = books.length - 1;

✓ After

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

class Stack {
  constructor() {
    this.top = null;
  }
  pop() {
    if (!this.top) return null;
    let popped = this.top.value;
    this.top = this.top.next;
    return popped;
  }
}

What It Enables

This lets you remove the last added item instantly, making your program faster and safer.

Real Life Example

When you undo typing in a text editor, it quickly removes the last action using a pop operation on a linked list stack.

Key Takeaways

Manual removal is slow and error-prone.

Pop operation removes the top node quickly.

Linked list tracks the top for instant access.

Practice

1. What does the pop() operation do in a linked list implemented using nodes?

easy

A. Adds a new node at the end

B. Returns the value of the last node without removing it

C. Removes the first node and returns its value

D. Removes the last node and returns its value

Solution

1. Step 1: Understand pop operation in linked list

   The pop operation removes the last node from the linked list and returns its value.

2. Step 2: Differentiate from other operations

   Adding a node is push, removing first node is shift, and peeking returns value without removal.

3. Final Answer:

   Removes the last node and returns its value -> Option D

4. Quick Check:

   pop() removes last node = B [OK]

Hint: Pop always removes the last node in linked list [OK]

Common Mistakes:

• Confusing pop with push (adding nodes)
• Thinking pop removes the first node
• Assuming pop only returns value without removal

2. Which of the following is the correct syntax to remove the last node in a singly linked list in JavaScript?

easy

A. list.popNode()

B. list.deleteTail()

C. list.pop()

D. list.removeLast()

Solution

1. Step 1: Identify standard method name

   In JavaScript linked list implementations, pop() is the standard method to remove the last node.

2. Step 2: Check other options

   Other method names like popNode(), removeLast(), or deleteTail() are not standard or common.

3. Final Answer:

   list.pop() -> Option C

4. Quick Check:

   pop() removes last node syntax = D [OK]

Hint: Use pop() to remove last node in linked list [OK]

Common Mistakes:

• Using non-standard method names
• Confusing pop() with other removal methods
• Assuming popNode() is built-in

3. Consider the following linked list and pop operation in JavaScript:

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

class LinkedList {
  constructor() {
    this.head = null;
  }
  push(value) {
    const newNode = new Node(value);
    if (!this.head) {
      this.head = newNode;
      return;
    }
    let current = this.head;
    while (current.next) {
      current = current.next;
    }
    current.next = newNode;
  }
  pop() {
    if (!this.head) return null;
    if (!this.head.next) {
      const val = this.head.value;
      this.head = null;
      return val;
    }
    let current = this.head;
    while (current.next.next) {
      current = current.next;
    }
    const val = current.next.value;
    current.next = null;
    return val;
  }
}

const list = new LinkedList();
list.push(10);
list.push(20);
list.push(30);
const poppedValue = list.pop();
console.log(poppedValue);

What will be printed?

medium

A. 30

B. 20

C. 10

D. null

Solution

1. Step 1: Trace push operations

   Nodes added in order: 10 -> 20 -> 30

2. Step 2: Trace pop operation

   Pop removes last node (30) and returns its value.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   Last node value popped = 30 [OK]

Hint: Pop returns last pushed value in linked list [OK]

Common Mistakes:

• Confusing pop with removing first node
• Expecting 20 instead of 30
• Assuming pop returns null if list not empty

4. The following pop() method for a singly linked list has a bug. What is the error?

pop() {
  if (!this.head) return null;
  let current = this.head;
  while (current.next) {
    current = current.next;
  }
  const val = current.value;
  current = null;
  return val;
}

medium

A. It does not update the previous node's next to null

B. It does not handle empty list

C. It returns the wrong node's value

D. It causes infinite loop

Solution

1. Step 1: Analyze traversal to last node

   The code moves current to last node correctly but does not track previous node.

2. Step 2: Check removal logic

   Setting current = null only changes local variable, does not remove last node from list.

3. Step 3: Identify missing update

   Previous node's next should be set to null to remove last node.

4. Final Answer:

   It does not update the previous node's next to null -> Option A

5. Quick Check:

   Missing previous.next = null causes bug = A [OK]

Hint: Update previous node's next to null to remove last node [OK]

Common Mistakes:

• Not tracking previous node during traversal
• Setting current = null does not remove node
• Ignoring empty list check

5. You have a singly linked list with nodes: 5 -> 15 -> 25 -> 35 -> null.
You perform two pop operations in a row. What is the state of the linked list after these operations?

hard

A. 15 -> 25 -> 35 -> null

B. 5 -> 15 -> null

C. 5 -> 15 -> 25 -> null

D. null

Solution

1. Step 1: Initial list nodes

   List is 5 -> 15 -> 25 -> 35 -> null.

2. Step 2: First pop removes last node

   Pop removes 35, list becomes 5 -> 15 -> 25 -> null.

3. Step 3: Second pop removes new last node

   Pop removes 25, list becomes 5 -> 15 -> null.

4. Final Answer:

   5 -> 15 -> null -> Option B

5. Quick Check:

   Two pops remove last two nodes = A [OK]

Hint: Each pop removes last node, so list shrinks from tail [OK]

Common Mistakes:

• Removing nodes from the front instead of end
• Removing only one node after two pops
• Assuming list becomes empty after two pops