Insert at Beginning of Doubly Linked List in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new node with the given data.

DSA Java

Node newNode = new Node([1]);

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

Anull

Bhead

Cdata

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'head' instead of 'data' to create the new node.

Passing 'null' or 'next' which are not data values.

We create a new node by passing the data value to the Node constructor.

2fill in blank

medium

Complete the code to link the new node's next pointer to the current head.

DSA Java

newNode.next = [1];

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

Anull

Bhead

CnewNode

DnewNode.prev

Attempts:

3 left

💡 Hint

Common Mistakes

Setting newNode.next to itself or null.

Confusing prev and next pointers.

The new node's next should point to the current head node.

3fill in blank

hard

Fix the error in setting the previous pointer of the old head node.

DSA Java

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

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

Aprev

Bnext

Cdata

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Setting head.next instead of head.prev.

Trying to set head.data or head itself.

The old head's previous pointer should point back to the new node.

4fill in blank

hard

Fill both blanks to update the head pointer and the new node's previous pointer.

DSA Java

head = [1];
[2].prev = null;

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

AnewNode

Bhead

Attempts:

3 left

💡 Hint

Common Mistakes

Setting head.prev to null instead of newNode.prev.

Using head instead of newNode in the second line.

The head pointer is updated to the new node, and the new node's previous pointer is set to null because it is now the first node.

5fill in blank

hard

Fill all three blanks to complete the insertAtBeginning method for a doubly linked list.

DSA Java

public void insertAtBeginning(int data) {
    Node newNode = new Node([1]);
    newNode.next = [2];
    if (head != null) {
        head.prev = newNode;
    }
    head = [3];
    newNode.prev = null;
}

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

Adata

Bhead

CnewNode

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up newNode and head in assignments.

Forgetting to set newNode.prev to null.

We create a new node with data, link its next to the current head, update the old head's prev pointer, then update head to newNode and set newNode.prev to null.

Practice

1. What is the main purpose of inserting a node at the beginning of a doubly linked list?

easy

A. To add a new node after the tail node

B. To reverse the entire list

C. To delete the first node of the list

D. To add a new node before the current head node

Solution

1. Step 1: Understand the position of insertion

   Inserting at the beginning means placing the new node before the current head node.

2. Step 2: Identify the effect on the list

   The new node becomes the new head, and its next pointer points to the old head.

3. Final Answer:

   To add a new node before the current head node -> Option D

4. Quick Check:

   Insert at beginning = add before head [OK]

Hint: Beginning insertion means new node becomes the head [OK]

Common Mistakes:

• Confusing beginning with end insertion
• Thinking insertion deletes nodes
• Mixing up singly and doubly linked list behavior

2. Which of the following is the correct syntax to create a new node in Java for a doubly linked list?

easy

A. Node newNode = Node(data); newNode.prev = null; newNode.next = null;

B. Node newNode = new Node(data); newNode.prev = null; newNode.next = null;

C. Node newNode = new Node(); newNode.data = data; newNode.prev = null; newNode.next = null;

D. Node newNode = new Node(data); newNode.prev = data; newNode.next = data;

Solution

1. Step 1: Understand Java object creation syntax

   In Java, objects are created using the 'new' keyword followed by the constructor.

2. Step 2: Check node initialization

   The node's previous and next pointers should be set to null initially.

3. Final Answer:

   Node newNode = new Node(data); newNode.prev = null; newNode.next = null; -> Option B

4. Quick Check:

   Java new object = new Constructor() [OK]

Hint: Use 'new' keyword and constructor to create objects [OK]

Common Mistakes:

• Omitting 'new' keyword
• Assigning data to prev or next pointers
• Using incorrect constructor syntax

3. What will be the output of the doubly linked list after inserting 10 at the beginning of the list containing nodes 20 <-> 30?

medium

A. 10 <-> 20 <-> 30

B. 20 <-> 30 <-> 10

C. 30 <-> 20 <-> 10

D. 10 <-> 30 <-> 20

Solution

1. Step 1: Identify current list order

   The list currently is 20 <-> 30, with 20 as head and 30 as tail.

2. Step 2: Insert 10 at beginning

   New node 10 becomes head, its next points to 20, and 20's prev points to 10.

3. Final Answer:

   10 <-> 20 <-> 30 -> Option A

4. Quick Check:

   Insert at beginning adds node before head [OK]

Hint: New node at start shifts old head right [OK]

Common Mistakes:

• Appending instead of prepending
• Not updating previous pointer of old head
• Reversing node order incorrectly

4. Given the following code snippet for inserting at the beginning of a doubly linked list, what is the error?

void insertAtBeginning(int data) {
  Node newNode = new Node(data);
  newNode.next = head;
  head.prev = newNode;
  head = newNode;
}

medium

A. No error, code works correctly

B. head should be updated before setting newNode.next

C. head.prev = newNode; causes NullPointerException if list is empty

D. newNode.next should be null

Solution

1. Step 1: Analyze code for empty list case

   If the list is empty, head is null, so head.prev causes NullPointerException.

2. Step 2: Identify fix

   Check if head is not null before setting head.prev = newNode.

3. Final Answer:

   head.prev = newNode; causes NullPointerException if list is empty -> Option C

4. Quick Check:

   Null head means head.prev causes error [OK]

Hint: Check for null head before accessing head.prev [OK]

Common Mistakes:

• Ignoring empty list case
• Assuming head is never null
• Misordering pointer updates

5. You have a doubly linked list with nodes: 5 <-> 15 <-> 25. You want to insert a new node with value 1 at the beginning and then reverse the list. What will be the final list after these operations?

hard

A. 25 <-> 15 <-> 5 <-> 1

B. 1 <-> 5 <-> 15 <-> 25

C. 1 <-> 25 <-> 15 <-> 5

D. 5 <-> 15 <-> 25 <-> 1

Solution

1. Step 1: Insert 1 at beginning

   List becomes 1 <-> 5 <-> 15 <-> 25 after insertion.

2. Step 2: Reverse the list

   Reversing swaps head and tail, so list becomes 25 <-> 15 <-> 5 <-> 1.

3. Final Answer:

   25 <-> 15 <-> 5 <-> 1 -> Option A

4. Quick Check:

   Insert at start then reverse flips order [OK]

Hint: Insert first, then reverse flips entire list [OK]

Common Mistakes:

• Reversing before insertion
• Not updating prev pointers when reversing
• Confusing node order after reverse