Dequeue Using Linked List in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Dequeue (Double-Ended Queue)?

A Dequeue is a data structure where elements can be added or removed from both the front and the rear ends.

beginner

Why use a linked list to implement a Dequeue?

A linked list allows efficient insertion and deletion at both ends without shifting elements, making it ideal for Dequeue operations.

beginner

What are the main operations of a Dequeue?

The main operations are: InsertFront, InsertRear, DeleteFront, DeleteRear, and Display.

intermediate

In a linked list based Dequeue, what pointers are typically maintained?

Two pointers: one to the front node and one to the rear node, to allow quick access for insertions and deletions.

beginner

What happens when you delete from an empty Dequeue?

An underflow condition occurs, meaning there are no elements to delete, so the operation should handle this gracefully.

Which operation is NOT valid in a Dequeue?

A Insert at rear

B Delete from middle

C Insert at front

D Delete from front

Dequeue supports insertion and deletion only at the front and rear ends, not from the middle.

What is the time complexity of insertion at front in a linked list based Dequeue?

A O(log n)

B O(n)

C O(1)

D O(n^2)

Insertion at front in a linked list is done in constant time O(1) by adjusting pointers.

Which pointers are essential in a linked list Dequeue?

A Only front pointer

B No pointers needed

C Only rear pointer

D Both front and rear pointers

Both front and rear pointers are needed to efficiently insert and delete at both ends.

What does 'underflow' mean in the context of Dequeue?

A Queue is empty

B Queue is sorted

C Queue has one element

D Queue is full

Underflow means trying to delete from an empty Dequeue.

Which data structure is best suited for implementing a Dequeue for dynamic size?

A Linked List

B Stack

C Array

D Binary Tree

Linked list allows dynamic size and efficient insertions/deletions at both ends.

Explain how insertion at the rear end works in a linked list based Dequeue.

Think about how the rear pointer moves and how the new node connects.

Describe the steps to delete an element from the front of a linked list based Dequeue.

Consider what happens when the Dequeue becomes empty after deletion.

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