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

Practice - 5 Tasks

1fill in blank

easy

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

DSA C#

Node newNode = new Node([1]);

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

Adata

Bvalue

CnodeData

Ditem

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that is not defined like 'value' or 'item'.

Forgetting to pass any argument to the Node constructor.

The new node should be created with the given data variable named 'data'.

2fill in blank

medium

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

DSA C#

newNode.[1] = head;

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

Aprev

Blink

Cnext

Dpointer

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' instead of 'next' for the forward link.

Using an undefined property like 'link' or 'pointer'.

The new node's next pointer 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 C#

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

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

Apointer

Bprev

Cnext

Dlink

Attempts:

3 left

💡 Hint

Common Mistakes

Setting the old head's next pointer instead of prev.

Using an incorrect property name like 'pointer' or 'link'.

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 C#

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

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

AnewNode

Bhead

Cprev

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Setting head to old head instead of new node.

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

The head should be updated to the new node, and the new node's prev pointer should be null as it is the first node.

5fill in blank

hard

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

DSA C#

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

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

Adata

Bnext

Cprev

Dpointer

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up next and prev pointers.

Not updating head to the new node.

Forgetting to set newNode.prev to null.

The method creates a new node with data, sets its next to head, updates old head's prev to new node, sets new node's prev to null, and updates head.

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 as the first element of the list

B. To remove the last node from the list

C. To search for a node in the list

D. To reverse the entire list

Solution

1. Step 1: Understand the operation

   Inserting at the beginning means adding a new node before the current first node.

2. Step 2: Identify the effect on the list

   The new node becomes the head (first node), shifting others to the right.

3. Final Answer:

   To add a new node as the first element of the list -> Option A

4. Quick Check:

   Insert at beginning = Add first node [OK]

Hint: Insert at beginning means new node becomes the first [OK]

Common Mistakes:

• Confusing insertion with deletion
• Thinking insertion adds at the end
• Mixing up search with insertion

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

easy

A. Node newNode = new Node(data);

B. newNode = Node(data);

C. Node newNode = Node(data);

D. Node newNode = new Node[];

Solution

1. Step 1: Recall C# object creation syntax

   In C#, objects are created using the 'new' keyword followed by the constructor call.

2. Step 2: Match syntax with node creation

   Correct syntax is 'Node newNode = new Node(data);' to create a new node with data.

3. Final Answer:

   Node newNode = new Node(data); -> Option A

4. Quick Check:

   C# new object = new ClassName() [OK]

Hint: Use 'new' keyword to create objects in C# [OK]

Common Mistakes:

• Omitting 'new' keyword
• Using array syntax for single node
• Missing type declaration

3. Consider this code snippet inserting a node with value 10 at the beginning of a doubly linked list:

Node newNode = new Node(10);
newNode.next = head;
if (head != null) {
    head.prev = newNode;
}
head = newNode;

What will be the printed list after insertion if the original list was: 20 <-> 30 <-> null?

medium

A. 30 <-> 20 <-> 10 <-> null

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

C. 10 <-> 30 <-> 20 <-> null

D. 10 <-> 20 <-> 30 <-> null

Solution

1. Step 1: Insert new node at beginning

   New node with 10 points next to old head (20). Old head's prev points back to new node.

2. Step 2: Update head pointer

   Head now points to new node (10), so list starts with 10.

3. Final Answer:

   10 <-> 20 <-> 30 <-> null -> Option D

4. Quick Check:

   New node at front = 10 first [OK]

Hint: New node points to old head; update old head's prev [OK]

Common Mistakes:

• Not updating old head's prev pointer
• Appending new node at end instead of beginning
• Not updating head to new node

4. Identify the error in this code snippet for inserting at the beginning of a doubly linked list:

Node newNode = new Node(5);
newNode.next = head;
head.prev = newNode;
head = newNode;

medium

A. Incorrect assignment of newNode.next

B. Missing check if head is null before accessing head.prev

C. head should be assigned before setting newNode.next

D. newNode.prev should be set to head

Solution

1. Step 1: Analyze head.prev assignment

   Code sets head.prev = newNode without checking if head is null, causing error if list is empty.

2. Step 2: Identify missing null check

   Must check if head != null before setting head.prev to avoid null reference error.

3. Final Answer:

   Missing check if head is null before accessing head.prev -> Option B

4. Quick Check:

   Check head != null before head.prev [OK]

Hint: Always check head != null before accessing head.prev [OK]

Common Mistakes:

• Ignoring null check on head
• Setting pointers in wrong order
• Confusing prev and next pointers

5. You have a doubly linked list with nodes: 5 <-> 15 <-> 25 <-> null. You want to insert a new node with value 1 at the beginning. After insertion, which of the following correctly represents the list and the pointers of the new node?

hard

A. 5 <-> 15 <-> 25 <-> 1 <-> null, newNode.prev = 25, newNode.next = null

B. 1 <-> 5 <-> 15 <-> 25 <-> null, newNode.prev = 5, newNode.next = null

C. 1 <-> 5 <-> 15 <-> 25 <-> null, newNode.prev = null, newNode.next = 5

D. 5 <-> 15 <-> 25 <-> 1 <-> null, newNode.prev = null, newNode.next = 5

Solution

1. Step 1: Insert new node at beginning

   New node with value 1 becomes head, points next to old head (5), prev is null.

2. Step 2: Confirm pointers and list order

   List order is 1 <-> 5 <-> 15 <-> 25 <-> null, newNode.prev = null, newNode.next = 5.

3. Final Answer:

   1 <-> 5 <-> 15 <-> 25 <-> null, newNode.prev = null, newNode.next = 5 -> Option C

4. Quick Check:

   New head prev=null, next=old head [OK]

Hint: New head prev=null, next=old head node [OK]

Common Mistakes:

• Setting newNode.prev to old head
• Appending new node at end instead of beginning
• Not updating head pointer