Delete from Beginning of Doubly Linked List in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to update the head pointer when deleting the first node.

DSA Java

head = [1];

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

Ahead

Bhead.prev

Cnull

Dhead.next

Attempts:

3 left

💡 Hint

Common Mistakes

Setting head to null without checking if list is empty

Using head.prev instead of head.next

When deleting the first node, the head should point to the next node.

2fill in blank

medium

Complete the code to set the previous pointer of the new head to null after deletion.

DSA Java

if (head != null) {
    head.[1] = null;
}

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

Aprev

Bdata

Cnext

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Setting next to null instead of prev

Not checking if head is null before setting prev

After deleting the first node, the new head's previous pointer should be null.

3fill in blank

hard

Fix the error in the code that deletes the first node of the doubly linked list.

DSA Java

public void deleteFromBeginning() {
    if (head == null) {
        return;
    }
    head = [1];
    if (head != null) {
        head.prev = null;
    }
}

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

Ahead.next

Bhead.prev

Cnull

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using head.prev which points backward

Setting head to null unconditionally

The head should be updated to head.next to remove the first node.

4fill in blank

hard

Fill both blanks to complete the method that deletes the first node and returns its data.

DSA Java

public int deleteFromBeginning() {
    if (head == null) {
        return -1;
    }
    int data = head.data;
    head = [1];
    if (head != null) {
        head.[2] = null;
    }
    return data;
}

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

Ahead.next

Bprev

Cnext

Dhead.prev

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping next and prev pointers

Not returning the deleted node's data

Update head to head.next and set head.prev to null after deletion.

5fill in blank

hard

Fill all three blanks to complete the method that deletes the first node and updates the list correctly.

DSA Java

public int deleteFromBeginning() {
    if (head == null) {
        return -1;
    }
    int data = head.[1];
    head = [2];
    if (head != null) {
        head.[3] = null;
    }
    return data;
}

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

Adata

Bhead.next

Cprev

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong field names

Not checking if head is null before setting prev

Access data from head.data, move head to head.next, and set head.prev to 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