Pop Using Linked List Node in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after popping the last node?

Given the linked list and the pop operation below, what will be the printed linked list after popping the last node?

DSA Javascript

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

class LinkedList {
  constructor() {
    this.head = null;
  }

  append(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;
  }

  print() {
    let current = this.head;
    let result = '';
    while (current) {
      result += current.value + ' -> ';
      current = current.next;
    }
    result += 'null';
    console.log(result);
  }
}

const list = new LinkedList();
list.append(10);
list.append(20);
list.append(30);
list.pop();
list.print();

A 20 -> 30 -> null

B 10 -> 20 -> 30 -> null

C 10 -> 20 -> null

D 10 -> null

💡 Hint

Remember that pop removes the last node from the linked list.

The pop method removes the last node (30) from the list. After popping, the list contains 10 and 20 nodes only.

❓ Predict Output

intermediate

What is the output after popping from a single-node list?

What will be the printed linked list after popping the only node in the list?

DSA Javascript

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

class LinkedList {
  constructor() {
    this.head = null;
  }

  append(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;
  }

  print() {
    let current = this.head;
    let result = '';
    while (current) {
      result += current.value + ' -> ';
      current = current.next;
    }
    result += 'null';
    console.log(result);
  }
}

const list = new LinkedList();
list.append(100);
list.pop();
list.print();

A Error: Cannot pop from empty list

B 100 -> null

C 100

D null

💡 Hint

Popping the only node should leave the list empty.

After popping the only node (100), the list becomes empty, so printing shows 'null'.

❓ Predict Output

advanced

What is the output after multiple pops on a multi-node list?

Consider the following code. What will be the printed linked list after popping twice?

DSA Javascript

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

class LinkedList {
  constructor() {
    this.head = null;
  }

  append(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;
  }

  print() {
    let current = this.head;
    let result = '';
    while (current) {
      result += current.value + ' -> ';
      current = current.next;
    }
    result += 'null';
    console.log(result);
  }
}

const list = new LinkedList();
list.append(1);
list.append(2);
list.append(3);
list.append(4);
list.pop();
list.pop();
list.print();

A 1 -> 2 -> null

B 2 -> 3 -> null

C 1 -> null

D 1 -> 2 -> 3 -> null

💡 Hint

Each pop removes the last node.

After popping twice, nodes 4 and 3 are removed, leaving 1 and 2.

❓ Predict Output

advanced

What error occurs when popping from an empty list?

What happens when pop is called on an empty linked list?

DSA Javascript

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

class LinkedList {
  constructor() {
    this.head = null;
  }

  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;
  }

  print() {
    let current = this.head;
    let result = '';
    while (current) {
      result += current.value + ' -> ';
      current = current.next;
    }
    result += 'null';
    console.log(result);
  }
}

const list = new LinkedList();
const popped = list.pop();
console.log(popped);

A null

B undefined

C TypeError: Cannot read property 'next' of null

D Error: List is empty

💡 Hint

Check the pop method's first condition.

The pop method returns null if the list is empty, so console.log prints null.

🧠 Conceptual

expert

How many nodes remain after popping all nodes one by one?

If you start with a linked list of 5 nodes and pop nodes one by one until the list is empty, how many nodes remain after the last pop?

A 1

B 0

C 5

D Cannot determine without code

💡 Hint

Popping removes one node each time until none remain.

Popping all nodes removes them all, so zero nodes remain at the end.

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