Pop Using Linked List Node in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to remove the last node from the linked list.

DSA Javascript

pop() {
  if (!this.head) return null;
  if (!this.head.next) {
    const poppedNode = this.head;
    this.head = null;
    return poppedNode;
  }
  let current = this.head;
  while (current.next.next) {
    current = current.[1];
  }
  const poppedNode = current.next;
  current.next = null;
  return poppedNode;
}

Drag options to blanks, or click blank then click option'

Atail

Bprev

Chead

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' instead of 'next' to move forward.

Trying to access 'head' or 'tail' properties inside the loop.

To reach the second last node, we move current to current.next repeatedly until current.next.next is null.

2fill in blank

medium

Complete the code to update the linked list after popping the last node.

DSA Javascript

pop() {
  if (!this.head) return null;
  if (!this.head.next) {
    const poppedNode = this.head;
    this.head = null;
    return poppedNode;
  }
  let current = this.head;
  while (current.next.next) {
    current = current.next;
  }
  const poppedNode = current.next;
  current.[1] = null;
  return poppedNode;
}

Drag options to blanks, or click blank then click option'

Anext

Bprev

Chead

Dtail

Attempts:

3 left

💡 Hint

Common Mistakes

Setting current.prev to null instead of current.next.

Trying to set head or tail to null incorrectly.

Setting current.next to null removes the last node from the list.

3fill in blank

hard

Fix the error in the pop method to correctly handle the empty list case.

DSA Javascript

pop() {
  if (this.head === [1]) return null;
  // rest of the code
}

Drag options to blanks, or click blank then click option'

Anull

Bundefined

C0

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for undefined instead of null.

Checking for 0 or false which are not valid for head.

The head is null when the list is empty, so we check if this.head === null.

4fill in blank

hard

Fill both blanks to complete the pop method that returns the popped node's value.

DSA Javascript

pop() {
  if (!this.head) return null;
  if (!this.head.next) {
    const poppedNode = this.head;
    this.head = null;
    return poppedNode.[1];
  }
  let current = this.head;
  while (current.next.next) {
    current = current.next;
  }
  const poppedNode = current.next;
  current.[2] = null;
  return poppedNode.value;
}

Drag options to blanks, or click blank then click option'

Avalue

Bnext

Cprev

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Returning the node instead of its value.

Setting current.prev to null instead of current.next.

Return the value of the popped node and set current.next to null to remove it.

5fill in blank

hard

Fill all three blanks to complete the pop method that updates tail and returns the popped node.

DSA Javascript

pop() {
  if (!this.head) return null;
  if (!this.head.next) {
    const poppedNode = this.head;
    this.head = null;
    this.[1] = null;
    return poppedNode;
  }
  let current = this.head;
  while (current.next.next) {
    current = current.next;
  }
  const poppedNode = current.next;
  current.next = null;
  this.[2] = current;
  return poppedNode;
  // this.[3] is updated to the new tail
}

Drag options to blanks, or click blank then click option'

Atail

Bhead

Cnext

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Updating head instead of tail.

Not updating tail after popping.

The tail pointer must be updated to current after popping the last node.

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