Delete from Beginning of Doubly Linked List in DSA Java - Build from Scratch

Delete from Beginning of Doubly Linked List

📖 Scenario: You are managing a playlist of songs using a doubly linked list. Each song is a node with a title and links to the previous and next songs. You want to remove the first song from the playlist when you no longer want to listen to it.

🎯 Goal: Build a doubly linked list with three songs, then delete the first song from the list, and finally print the updated playlist from start to end.

📋 What You'll Learn

Create a doubly linked list with exactly three nodes containing the song titles: "SongA", "SongB", and "SongC" in that order.

Create a variable called head that points to the first node of the list.

Write code to delete the first node from the doubly linked list by updating head and the links properly.

Print the titles of the songs in the updated list from beginning to end separated by arrows like: SongB -> SongC -> null

💡 Why This Matters

🌍 Real World

Doubly linked lists are used in music players, browsers, and other apps to move forward and backward through items efficiently.

💼 Career

Understanding linked list operations is fundamental for software developers working with data structures, memory management, and system design.

1

Create the doubly linked list with three songs

Create a class called Node with String title, Node prev, and Node next. Then create three Node objects with titles "SongA", "SongB", and "SongC". Link them to form a doubly linked list in that order. Finally, create a variable called head that points to the first node.

DSA Java

// Define Node class and create three nodes linked as doubly linked list
// Set head to the first node
// Your code here

Hint

Remember to link each node's next and prev pointers correctly to form the doubly linked list.

2

Add a variable to hold the updated head after deletion

Create a variable called newHead and set it to the node after head (the second node in the list).

DSA Java

class Node {
    String title;
    Node prev;
    Node next;

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

public class Main {
    public static void main(String[] args) {
        Node node1 = new Node("SongA");
        Node node2 = new Node("SongB");
        Node node3 = new Node("SongC");

        node1.next = node2;
        node2.prev = node1;
        node2.next = node3;
        node3.prev = node2;

        Node head = node1;
        // Create newHead as the node after head
        // Your code here
    }
}

Hint

Set newHead to the node that comes after head.

3

Delete the first node by updating links

Set newHead.prev to null to remove the backward link to the old first node. Then update head to point to newHead.

DSA Java

class Node {
    String title;
    Node prev;
    Node next;

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

public class Main {
    public static void main(String[] args) {
        Node node1 = new Node("SongA");
        Node node2 = new Node("SongB");
        Node node3 = new Node("SongC");

        node1.next = node2;
        node2.prev = node1;
        node2.next = node3;
        node3.prev = node2;

        Node head = node1;
        Node newHead = head.next;
        // Remove backward link from newHead
        // Update head to newHead
        // Your code here
    }
}

Hint

Set the prev of the new head to null and then assign head to newHead.

4

Print the updated doubly linked list from beginning to end

Use a Node variable called current starting at head. Use a while loop to print each node's title followed by " -> ". After the loop, print null to show the end of the list.

DSA Java

class Node {
    String title;
    Node prev;
    Node next;

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

public class Main {
    public static void main(String[] args) {
        Node node1 = new Node("SongA");
        Node node2 = new Node("SongB");
        Node node3 = new Node("SongC");

        node1.next = node2;
        node2.prev = node1;
        node2.next = node3;
        node3.prev = node2;

        Node head = node1;
        Node newHead = head.next;
        newHead.prev = null;
        head = newHead;

        // Print the list from head to end
        // Your code here
    }
}

Hint

Use a loop to move through the list starting at head and print each title followed by " -> ". After the loop, print null.

Practice

1. What happens to the head pointer when you delete the first node from a doubly linked list?

easy

A. It moves to the second node in the list

B. It becomes null immediately

C. It points to the last node

D. It remains unchanged

Solution

1. Step 1: Understand the head pointer role

   The head pointer always points to the first node in the doubly linked list.

2. Step 2: Effect of deleting the first node

   When the first node is deleted, the head pointer must move to the next node, which becomes the new first node.

3. Final Answer:

   It moves to the second node in the list -> Option A

4. Quick Check:

   Deleting first node updates head to next node [OK]

Hint: Head always points to first node; update it after deletion [OK]

Common Mistakes:

• Not updating head pointer after deletion
• Setting head to null even if list has more nodes
• Confusing head with tail pointer

2. Which of the following is the correct Java statement to update the prev pointer of the new head node after deleting the first node in a doubly linked list?

easy

A. head = null.prev;

B. head.next = null;

C. head.prev = null;

D. head.prev.next = null;

Solution

1. Step 1: Identify the new head node

   After deleting the first node, the new head is the second node in the list.

2. Step 2: Set the previous pointer of new head to null

   Since the new head is the first node now, its prev pointer must be null to indicate no node before it.

3. Final Answer:

   head.prev = null; -> Option C

4. Quick Check:

   New head prev pointer must be null [OK]

Hint: New head's prev pointer is always null [OK]

Common Mistakes:

• Setting head.next to null instead of head.prev
• Using invalid syntax like null.prev
• Trying to access prev.next which may cause error

3. Consider the following Java code snippet for deleting the first node from a doubly linked list:

if (head == null) return;
head = head.next;
if (head != null) head.prev = null;

What will be the state of the list after deleting the first node from the list 1 <-> 2 <-> 3?

medium

A. 1 <-> 3

B. Empty list

C. 3 only

D. 2 <-> 3

Solution

1. Step 1: Initial list and deletion

   The list is 1 <-> 2 <-> 3. Deleting the first node means removing node with value 1.

2. Step 2: Update head and prev pointer

   Head moves to node 2. Then head.prev is set to null, so node 2 has no previous node.

3. Final Answer:

   2 <-> 3 -> Option D

4. Quick Check:

   First node removed, head points to second node [OK]

Hint: After deletion, head points to second node, prev set null [OK]

Common Mistakes:

• Not setting head.prev to null causing broken links
• Assuming node 1 still exists
• Confusing next and prev pointers

4. The following code attempts to delete the first node from a doubly linked list but causes a runtime error:

if (head == null) return;
head = head.next;
head.prev = null;

What is the error and how to fix it?

medium

A. Syntax error due to missing semicolon; add semicolon after head.prev = null

B. NullPointerException when list has one node; fix by checking if head is null before setting head.prev

C. Logical error: head should not be updated; remove head = head.next

D. No error; code works fine

Solution

1. Step 1: Identify cause of NullPointerException

   If the list has only one node, after head = head.next, head becomes null. Then head.prev causes NullPointerException.

2. Step 2: Fix by checking head before accessing prev

   Add a condition to check if head is not null before setting head.prev = null.

3. Final Answer:

   NullPointerException when list has one node; fix by checking if head is null before setting head.prev -> Option B

4. Quick Check:

   Always check for null before accessing object members [OK]

Hint: Check head != null before head.prev = null to avoid errors [OK]

Common Mistakes:

• Not checking for null causing runtime errors
• Assuming list always has multiple nodes
• Ignoring exception details

5. You have a doubly linked list with nodes 10 <-> 20 <-> 30 <-> 40. After deleting the first node twice using the correct deletion method, what will be the printed list?

hard

A. 30 <-> 40

B. 20 <-> 30 <-> 40

C. 40 only

D. Empty list

Solution

1. Step 1: First deletion

   Deleting first node removes 10. Head moves to 20, head.prev set to null. List becomes 20 <-> 30 <-> 40.

2. Step 2: Second deletion

   Deleting first node again removes 20. Head moves to 30, head.prev set to null. List becomes 30 <-> 40.

3. Final Answer:

   30 <-> 40 -> Option A

4. Quick Check:

   Two deletions remove first two nodes [OK]

Hint: Each deletion moves head forward and updates prev to null [OK]

Common Mistakes:

• Deleting only once instead of twice
• Not updating head.prev after second deletion
• Printing old list without changes