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

Practice - 5 Tasks

1fill in blank

easy

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

DSA Javascript

class Node {
  constructor(data) {
    this.data = [1];
    this.prev = null;
    this.next = null;
  }
}

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

Anode

Bvalue

Cdata

Ditem

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name not defined in the constructor.

Assigning to this.value instead of this.data.

The constructor should assign the passed data to this.data to store the node's value.

2fill in blank

medium

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

DSA Javascript

function insertAtBeginning(head, data) {
  const newNode = new Node(data);
  newNode.next = [1];
  if (head !== null) {
    head.prev = newNode;
  }
  return newNode;
}

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

Ahead

Bdata

Cnull

DnewNode

Attempts:

3 left

💡 Hint

Common Mistakes

Setting newNode.next to null or data instead of head.

Setting newNode.next to newNode itself.

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

3fill in blank

hard

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

DSA Javascript

function insertAtBeginning(head, data) {
  const newNode = new Node(data);
  newNode.next = head;
  if (head !== null) {
    head.[1] = newNode;
  }
  return newNode;
}

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

Anext

Bprevious

Cpointer

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'next' instead of 'prev'.

Using incorrect property names like 'previous' or 'pointer'.

The previous pointer of the old head should point to the new node.

4fill in blank

hard

Fill both blanks to complete the function that inserts a node at the beginning of the doubly linked list.

DSA Javascript

function insertAtBeginning(head, data) {
  const newNode = new Node(data);
  newNode.[1] = head;
  if (head !== null) {
    head.[2] = newNode;
  }
  return newNode;
}

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

Anext

Bprev

Cprevious

Dpointer

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up 'next' and 'prev' pointers.

Using incorrect property names.

The new node's next points to the old head, and the old head's prev points back to the new node.

5fill in blank

hard

Fill all three blanks to complete the insertAtBeginning function with proper pointer updates.

DSA Javascript

function insertAtBeginning(head, data) {
  const newNode = new Node(data);
  newNode.[1] = head;
  newNode.[2] = null;
  if (head !== null) {
    head.[3] = newNode;
  }
  return newNode;
}

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

Anext

Bprev

Cprevious

Dpointer

Attempts:

3 left

💡 Hint

Common Mistakes

Not setting newNode.prev to null.

Not updating head.prev correctly.

Mixing up next and prev pointers.

The new node's next points to the old head, its prev is null because it becomes the first node, and the old head's prev points back to the new node.

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 before the current head node

B. To add a new node after the tail node

C. To remove the first node from the list

D. To reverse the entire list

Solution

1. Step 1: Understand the head node position

   The head node is the first node in the doubly linked list.

2. Step 2: Insert new node before head

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

3. Final Answer:

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

4. Quick Check:

   Insert at beginning = add before head [OK]

Hint: New node becomes new head in doubly linked list [OK]

Common Mistakes:

• Confusing insertion at beginning with insertion at end
• Thinking insertion removes nodes
• Mixing up head and tail nodes

2. Which of the following is the correct way to update the previous pointer of the old head node when inserting a new node at the beginning in JavaScript?

easy

A. oldHead.prev = newNode;

B. newNode.next = oldHead.prev;

C. newNode.prev = oldHead;

D. oldHead.next = newNode;

Solution

1. Step 1: Identify the old head's previous pointer

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

2. Step 2: Update old head's prev to new node

   Setting oldHead.prev = newNode links the old head back to the new node.

3. Final Answer:

   oldHead.prev = newNode; -> Option A

4. Quick Check:

   Old head prev points to new node [OK]

Hint: Old head's prev points to new node after insertion [OK]

Common Mistakes:

• Setting oldHead.next instead of prev
• Confusing newNode's pointers
• Assigning newNode.next incorrectly

3. What will be the printed state of the doubly linked list after inserting a node with value 10 at the beginning of the list 5 <-> 15 <-> 20?

medium

A. 5 <-> 15 <-> 20 <-> 10

B. 10 <-> 15 <-> 20 <-> 5

C. 10 <-> 5 <-> 15 <-> 20

D. 5 <-> 10 <-> 15 <-> 20

Solution

1. Step 1: Insert 10 before current head (5)

   The new node with value 10 becomes the new head, pointing next to 5.

2. Step 2: Link the rest of the list after 5

   The list continues as 5 <-> 15 <-> 20 unchanged after insertion.

3. Final Answer:

   10 <-> 5 <-> 15 <-> 20 -> Option C

4. Quick Check:

   New head 10 followed by old list [OK]

Hint: New node appears first, old list follows [OK]

Common Mistakes:

• Appending new node at end instead of beginning
• Swapping nodes incorrectly
• Ignoring pointer updates

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

function insertAtBeginning(head, data) {
  const newNode = { data: data, prev: null, next: head };
  head.prev = newNode;
  head = newNode;
  return head;
}

medium

A. It incorrectly sets newNode.next to null

B. It does not handle empty list when head is null

C. It forgets to update newNode.prev to head

D. It returns the old head instead of new head

Solution

1. Step 1: Check for empty list case

   If head is null, head.prev causes error because head is null.

2. Step 2: Missing null check before setting head.prev

   Code should check if head exists before setting head.prev = newNode.

3. Final Answer:

   It does not handle empty list when head is null -> Option B

4. Quick Check:

   Null head requires special handling [OK]

Hint: Always check if head is null before accessing its properties [OK]

Common Mistakes:

• Ignoring empty list case
• Assuming head is never null
• Not returning updated head

5. You have a doubly linked list with nodes: 7 <-> 14 <-> 21. You want to insert a new node with value 3 at the beginning. After insertion, what will be the correct sequence of node values when traversing from head to tail and tail to head?

hard

A. Head to tail: 21 <-> 14 <-> 7 <-> 3; Tail to head: 3 <-> 7 <-> 14 <-> 21

B. Head to tail: 7 <-> 14 <-> 21 <-> 3; Tail to head: 3 <-> 21 <-> 14 <-> 7

C. Head to tail: 3 <-> 7 <-> 14 <-> 21; Tail to head: 3 <-> 7 <-> 14 <-> 21

D. Head to tail: 3 <-> 7 <-> 14 <-> 21; Tail to head: 21 <-> 14 <-> 7 <-> 3

Solution

1. Step 1: Insert 3 before current head 7

   The new node 3 becomes the head, pointing next to 7.

2. Step 2: Traverse forward and backward

   Forward traversal: 3 -> 7 -> 14 -> 21; backward traversal reverses this order.

3. Final Answer:

   Head to tail: 3 <-> 7 <-> 14 <-> 21; Tail to head: 21 <-> 14 <-> 7 <-> 3 -> Option D

4. Quick Check:

   New head first, tail last, pointers reversed backward [OK]

Hint: Head to tail is forward order; tail to head is reverse [OK]

Common Mistakes:

• Mixing forward and backward traversal order
• Appending new node at tail instead of head
• Incorrect pointer updates causing wrong traversal