Pop Using Linked List Node in DSA Java - Build from Scratch

Pop Using Linked List Node

📖 Scenario: You are managing a simple stack of books using a linked list. Each book is a node in the list. You want to remove the top book from the stack.

🎯 Goal: Build a linked list with three nodes representing books, then remove (pop) the top book node from the list, and finally print the remaining books in order.

📋 What You'll Learn

Create a linked list with three nodes named head, second, and third.

Each node should hold a string book title: "Book1", "Book2", and "Book3" respectively.

Create a variable called top that points to the head node.

Write code to pop the top node from the linked list by moving top to the next node.

Print the titles of the remaining nodes starting from top.

💡 Why This Matters

🌍 Real World

Linked lists are used in many applications like undo functionality in text editors, managing playlists, and stacks in programming languages.

💼 Career

Understanding how to manipulate linked lists and perform stack operations like pop is fundamental for software development and technical interviews.

1

Create the linked list nodes

Create three linked list nodes named head, second, and third using the Node class. Assign the book titles "Book1", "Book2", and "Book3" to their data fields respectively. Link them so that head points to second, and second points to third. Set third.next to null.

DSA Java

class Node {
    String data;
    Node next;

    Node(String data) {
        this.data = data;
        this.next = null;
    }
}

public class Main {
    public static void main(String[] args) {
        // Create head node with data "Book1"
        // Create second node with data "Book2"
        // Create third node with data "Book3"
        // Link head.next to second
        // Link second.next to third
        // Set third.next to null
    }
}

Hint

Remember to create each node with the new Node() constructor and link them using the next field.

2

Create a pointer to the top node

Create a variable called top and set it to point to the head node.

DSA Java

class Node {
    String data;
    Node next;

    Node(String data) {
        this.data = data;
        this.next = null;
    }
}

public class Main {
    public static void main(String[] args) {
        Node head = new Node("Book1");
        Node second = new Node("Book2");
        Node third = new Node("Book3");

        head.next = second;
        second.next = third;
        third.next = null;

        // Create top pointer to head
    }
}

Hint

Just assign top to head to point to the first node.

3

Pop the top node

Pop the top node from the linked list by moving the top pointer to the next node. Write the code to set top to top.next.

DSA Java

class Node {
    String data;
    Node next;

    Node(String data) {
        this.data = data;
        this.next = null;
    }
}

public class Main {
    public static void main(String[] args) {
        Node head = new Node("Book1");
        Node second = new Node("Book2");
        Node third = new Node("Book3");

        head.next = second;
        second.next = third;
        third.next = null;

        Node top = head;

        // Pop the top node by moving top to top.next
    }
}

Hint

Set top equal to top.next to remove the first node.

4

Print the remaining nodes

Use a while loop to print the data of each node starting from top. Print each book title on its own line.

DSA Java

class Node {
    String data;
    Node next;

    Node(String data) {
        this.data = data;
        this.next = null;
    }
}

public class Main {
    public static void main(String[] args) {
        Node head = new Node("Book1");
        Node second = new Node("Book2");
        Node third = new Node("Book3");

        head.next = second;
        second.next = third;
        third.next = null;

        Node top = head;

        top = top.next;

        // Print remaining nodes starting from top
    }
}

Hint

Use a while loop to go through each node starting at top and print the data field.

Practice

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

easy

A. Removes and returns the first node of the linked list

B. Adds a new node at the end of the linked list

C. Removes the last node without returning it

D. Returns the size of the linked list

Solution

1. Step 1: Understand pop operation in linked list context

   The pop operation removes the top element, which is the first node in a linked list acting like a stack.

2. Step 2: Identify what is returned

   Pop returns the value of the removed first node and updates the head pointer to the next node.

3. Final Answer:

   Removes and returns the first node of the linked list -> Option A

4. Quick Check:

   Pop = Remove first node [OK]

Hint: Pop removes the head node and returns it [OK]

Common Mistakes:

• Thinking pop removes the last node
• Confusing pop with push operation
• Assuming pop returns list size

2. Which of the following is the correct Java syntax to pop a node from the head of a singly linked list?

easy

A. head = head.next; return head.data;

B. Node temp = head; head = head.next; return temp.data;

C. Node temp = head.next; head = temp; return head.data;

D. return head.next.data; head = head.next;

Solution

1. Step 1: Check correct order of operations

   We must save the current head node in a temp variable before moving head to next.

2. Step 2: Return the data of the removed node

   Return the data from the saved temp node, not the new head.

3. Final Answer:

   Node temp = head; head = head.next; return temp.data; -> Option B

4. Quick Check:

   Save head, move head, return old data [OK]

Hint: Save head before moving it to next [OK]

Common Mistakes:

• Returning data from new head instead of old
• Not saving old head before moving
• Returning data before updating head

3. Given the linked list nodes: 10 -> 20 -> 30 -> null, what is the list after one pop operation using this code?

Node pop() {
  if (head == null) return null;
  Node temp = head;
  head = head.next;
  return temp;
}

medium

A. 30 -> null

B. 10 -> 20 -> 30 -> null

C. 20 -> 30 -> null

D. null

Solution

1. Step 1: Identify the node removed by pop()

   Pop removes the head node which has value 10.

2. Step 2: Update head to next node

   Head moves from 10 to 20, so list becomes 20 -> 30 -> null.

3. Final Answer:

   20 -> 30 -> null -> Option C

4. Quick Check:

   Pop removes first node 10 [OK]

Hint: Pop removes first node, head moves next [OK]

Common Mistakes:

• Assuming pop removes last node
• Not updating head pointer
• Returning old list unchanged

4. What is the error in this pop method for a singly linked list?

Node pop() {
  if (head == null) return null;
  Node temp = head.next;
  head = temp;
  return temp;
}

medium

A. It skips the first node and removes the second node instead

B. It causes a null pointer exception when list is empty

C. It returns the old head node instead of new head

D. It does not update the head pointer

Solution

1. Step 1: Analyze which node is assigned to temp

   Temp is assigned head.next, so the first node is skipped.

2. Step 2: Understand effect on head and return

   Head moves to second node, so first node is never removed; second node is removed instead.

3. Final Answer:

   It skips the first node and removes the second node instead -> Option A

4. Quick Check:

   Pop must remove first node, not second [OK]

Hint: Pop must save head, not head.next [OK]

Common Mistakes:

• Assigning temp to head.next instead of head
• Returning the new head instead of the old head
• Returning wrong node

5. You have a linked list: 5 -> 15 -> 25 -> 35 -> null. You perform two pop operations using this method:

Node pop() {
  if (head == null) return null;
  Node temp = head;
  head = head.next;
  return temp;
}

What is the state of the list after these two pops?

hard

A. null

B. 15 -> 25 -> 35 -> null

C. 35 -> null

D. 25 -> 35 -> null

Solution

1. Step 1: First pop removes 5

   After first pop, head moves from 5 to 15, list is 15 -> 25 -> 35 -> null.

2. Step 2: Second pop removes 15

   After second pop, head moves from 15 to 25, list is 25 -> 35 -> null.

3. Final Answer:

   25 -> 35 -> null -> Option D

4. Quick Check:

   Two pops remove first two nodes [OK]

Hint: Each pop removes head, moves head next [OK]

Common Mistakes:

• Removing last nodes instead of first
• Not updating head after each pop
• Confusing pop with push