Pop Using Linked List Node in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the 'pop' operation do in a linked list?

It removes the last node from the linked list and returns its value.

intermediate

In a singly linked list, why do we need to traverse the list to pop the last node?

Because each node only knows about the next node, we must find the second last node to update its next pointer to null.

beginner

What is the state of the linked list after popping the last node from a list with one node?

The linked list becomes empty (head is null).

intermediate

Show the steps to pop the last node from a linked list: 1 -> 2 -> 3 -> null

1. Start at head (1). 2. Move to next (2). 3. Move to next (3). 4. Since 3 is last, remove it by setting 2's next to null. 5. Return value 3. Result: 1 -> 2 -> null

beginner

What should the pop function return if the linked list is empty?

It should return null or undefined to indicate there is no node to pop.

What pointer do you update when popping the last node from a singly linked list?

A The next pointer of the second last node

B The head pointer

C The next pointer of the last node

D The previous pointer of the last node

You update the next pointer of the second last node to null to remove the last node.

What is the time complexity of popping the last node in a singly linked list?

A O(1)

B O(n)

C O(log n)

D O(n^2)

You must traverse the list to find the second last node, which takes O(n) time.

If a linked list has only one node, what happens when you pop it?

A The head points to itself

B The list remains the same

C The list becomes empty

D An error occurs

Popping the only node removes it, so the list becomes empty (head is null).

What should the pop function return if the list is empty?

A The last node's value

B An error message

C 0

D null or undefined

Since there is no node to pop, returning null or undefined is appropriate.

Which of these is NOT a step in popping the last node from a singly linked list?

A Update the head pointer to the last node

B Set the second last node's next pointer to null

C Traverse to the second last node

D Return the value of the removed node

You do not update the head pointer to the last node; you remove the last node by updating the second last node's next pointer.

Explain how to pop the last node from a singly linked list step-by-step.

Think about how to find the node before the last one.

Describe the edge cases to consider when implementing pop on a linked list.

What happens if the list has zero or one 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