Dequeue Using Linked List in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a new node at the front of the dequeue.

DSA C#

public void AddFront(int value) {
    Node newNode = new Node(value);
    if (front == null) {
        front = rear = newNode;
    } else {
        newNode.next = [1];
        front.prev = newNode;
        front = newNode;
    }
}

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

Afront

Brear

Cnull

DnewNode

Attempts:

3 left

💡 Hint

Common Mistakes

Setting newNode.next to rear instead of front

Forgetting to update front.prev

Assigning newNode.next to null

To add at the front, the new node's next should point to the current front node.

2fill in blank

medium

Complete the code to remove a node from the rear of the dequeue.

DSA C#

public int RemoveRear() {
    if (rear == null) throw new InvalidOperationException("Dequeue is empty");
    int value = rear.data;
    rear = rear.prev;
    if (rear != null) {
        rear.[1] = null;
    } else {
        front = null;
    }
    return value;
}

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

Adata

Bfront

Cprev

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Setting rear.prev to null instead of rear.next

Not updating front when rear becomes null

Returning wrong value

After moving rear back, its next pointer should be set to null to detach the removed node.

3fill in blank

hard

Fix the error in the code to add a node at the rear of the dequeue.

DSA C#

public void AddRear(int value) {
    Node newNode = new Node(value);
    if (rear == null) {
        front = rear = newNode;
    } else {
        rear.next = newNode;
        newNode.[1] = rear;
        rear = newNode;
    }
}

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

Afront

Bdata

Cprev

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Setting newNode.next instead of newNode.prev

Not updating rear pointer

Assigning newNode.front or newNode.data incorrectly

The new node's prev pointer should point to the old rear node.

4fill in blank

hard

Fill both blanks to correctly remove a node from the front of the dequeue.

DSA C#

public int RemoveFront() {
    if (front == null) throw new InvalidOperationException("Dequeue is empty");
    int value = front.data;
    front = front.[1];
    if (front != null) {
        front.[2] = null;
    } else {
        rear = null;
    }
    return value;
}

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

Anext

Bprev

Cfront

Drear

Attempts:

3 left

💡 Hint

Common Mistakes

Using prev instead of next to move front

Not clearing front.prev after removal

Not updating rear when list becomes empty

To remove front, move front to front.next and set new front.prev to null.

5fill in blank

hard

Fill all three blanks to create a method that checks if the dequeue is empty.

DSA C#

public bool IsEmpty() {
    return [1] == null && [2] == null && [3] == 0;
}

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

Afront

Brear

Ccount

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Checking size instead of count

Checking only front or rear but not both

Using incorrect variable names

The dequeue is empty if front and rear are null and count is zero.

Practice

1. What is the main advantage of implementing a Dequeue using a linked list compared to an array?

easy

A. It automatically sorts the elements.

B. It uses less memory than an array.

C. It allows insertion and deletion from both ends in constant time.

D. It prevents duplicate elements.

Solution

1. Step 1: Understand Dequeue operations

   A dequeue allows adding and removing elements from both front and rear ends.

2. Step 2: Compare linked list vs array

   Linked list supports constant time insertion and deletion at both ends, unlike arrays which may require shifting elements.

3. Final Answer:

   It allows insertion and deletion from both ends in constant time. -> Option C

4. Quick Check:

   Linked list dequeue = O(1) insert/delete both ends [OK]

Hint: Linked list supports fast add/remove at both ends [OK]

Common Mistakes:

• Thinking arrays allow O(1) insertion at front
• Confusing memory usage advantage
• Assuming automatic sorting

2. Which of the following is the correct way to declare a node class for a doubly linked list in C# for implementing a dequeue?

easy

A. public class Node { int data; Node next; }

B. public class Node { public int data; public Node next; public Node prev; }

C. class Node { public int data; Node next; Node prev; }

D. public struct Node { int data; Node next; Node prev; }

Solution

1. Step 1: Check access modifiers and fields

   For linked list nodes, fields should be public to access from dequeue class.

2. Step 2: Verify presence of next and prev pointers

   Doubly linked list nodes need both next and prev references.

3. Final Answer:

   public class Node { public int data; public Node next; public Node prev; } -> Option B

4. Quick Check:

   Public fields + next + prev = correct node class [OK]

Hint: Node needs public data, next, and prev for dequeue [OK]

Common Mistakes:

• Missing public keyword for fields
• Using struct instead of class
• Omitting prev pointer

3. Consider the following C# code snippet implementing a dequeue using a linked list. What will be printed after these operations?

Dequeue dq = new Dequeue();
dq.InsertFront(10);
dq.InsertRear(20);
dq.InsertFront(5);
dq.DeleteRear();
dq.Display();

medium

A. 5 -> 10 -> 20 -> null

B. 10 -> 5 -> null

C. 10 -> 20 -> null

D. 5 -> 10 -> null

Solution

1. Step 1: Trace insertions

   InsertFront(10) adds 10 at front: 10 -> null InsertRear(20) adds 20 at rear: 10 -> 20 -> null InsertFront(5) adds 5 at front: 5 -> 10 -> 20 -> null

2. Step 2: Trace deletion and display

   DeleteRear() removes 20 from rear: 5 -> 10 -> null Display prints elements from front to rear.

3. Final Answer:

   5 -> 10 -> null -> Option D

4. Quick Check:

   After delete rear, list = 5 -> 10 [OK]

Hint: Track insert/delete order carefully [OK]

Common Mistakes:

• Forgetting delete rear removes last element
• Mixing front and rear insertions
• Incorrect display order

4. The following C# method is intended to delete the front node of a dequeue implemented using a doubly linked list. What is the error?

public void DeleteFront() {
    if (front == null) return;
    front = front.next;
    front.prev = null;
}

medium

A. It does not handle the case when the list becomes empty after deletion.

B. It should update rear pointer instead of front.

C. It should set front.next to null before updating.

D. It should not set front.prev to null.

Solution

1. Step 1: Analyze deletion steps

   After moving front to front.next, if front becomes null (list empty), front.prev = null causes NullReferenceException.

2. Step 2: Identify missing null check

   Code must check if front is null before setting front.prev = null to avoid error.

3. Final Answer:

   It does not handle the case when the list becomes empty after deletion. -> Option A

4. Quick Check:

   Null check needed after front update [OK]

Hint: Check for empty list after deletion [OK]

Common Mistakes:

• Ignoring null after front update
• Updating rear pointer incorrectly
• Setting pointers in wrong order

5. You want to implement a method ReverseDisplay() for a dequeue using a doubly linked list that prints elements from rear to front. Which approach is correct?

hard

A. Start from rear node and traverse using prev pointers until null, printing each node's data.

B. Start from front node and traverse using next pointers until null, printing each node's data.

C. Use recursion starting from front node to print in reverse order.

D. Swap front and rear pointers and then traverse from front using next pointers.

Solution

1. Step 1: Understand doubly linked list structure

   Each node has next and prev pointers; rear node's next is null, front node's prev is null.

2. Step 2: Reverse traversal logic

   To print from rear to front, start at rear and move backward using prev pointers until null.

3. Final Answer:

   Start from rear node and traverse using prev pointers until null, printing each node's data. -> Option A

4. Quick Check:

   Traverse backward from rear using prev [OK]

Hint: Traverse prev pointers from rear to front [OK]

Common Mistakes:

• Traversing next pointers from front for reverse
• Swapping pointers incorrectly
• Using recursion unnecessarily