Dequeue Using Linked List in DSA C# - Execution Trace

Concept Flow - Dequeue Using Linked List

Create new node

↓

Is Dequeue empty?

Yes→Set head and tail to new node

| No↓

Add node at tail (enqueue)

↓

Remove node from head (dequeue front)

↓

Remove node from tail (dequeue rear)

↓

Update head or tail pointers

↓

Return removed value or updated list

Shows how nodes are added at tail and removed from head or tail, updating pointers accordingly.

Execution Sample

DSA C#

Enqueue(10)
Enqueue(20)
Enqueue(30)
DequeueFront()
DequeueRear()

Adds 10, 20, 30 to the end, then removes from front and rear.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Enqueue 10	10	head -> 10, tail -> 10	10 -> null
2	Enqueue 20	10 -> 20	tail -> 20	10 -> 20 -> null
3	Enqueue 30	10 -> 20 -> 30	tail -> 30	10 -> 20 -> 30 -> null
4	DequeueFront	20 -> 30	head -> 20	20 -> 30 -> null
5	DequeueRear	20	tail -> 20	20 -> null
6	End	20	No change	20 -> null

💡 No more operations; list has one node left.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
head	null	Node(10)	Node(10)	Node(10)	Node(20)	Node(20)	Node(20)
tail	null	Node(10)	Node(20)	Node(30)	Node(30)	Node(20)	Node(20)
list size	0	1	2	3	2	1	1

Key Moments - 3 Insights

Why does DequeueFront update the head pointer but not the tail?

Because removing from the front means the head moves to the next node; tail stays same unless list becomes empty (see step 4 in execution_table).

What happens to the tail pointer when DequeueRear removes the last node?

Tail moves to the previous node; if list becomes empty, both head and tail become null. In step 5, tail moves from Node(30) to Node(20).

Why is the list size updated after each operation?

To keep track of how many nodes are in the list, helping to know when the list is empty or has one node (see variable_tracker).

Visual Quiz - 3 Questions

Look at the execution_table, what is the head pointer after step 3?

A Node(10)

B Node(20)

C Node(30)

D null

💡 Hint

Check the 'Pointer Changes' column at step 3 in execution_table.

At which step does the list size become 2?

A After step 1

B After step 2

C After step 4

D After step 5

💡 Hint

Look at the 'list size' row in variable_tracker after each step.

If we add Enqueue(40) after step 5, what will be the new tail node?

A Node(20)

B Node(30)

C Node(40)

D null

💡 Hint

Enqueue adds at tail; see how tail changes in execution_table steps 2 and 3.

Concept Snapshot

Dequeue Using Linked List:
- Enqueue adds node at tail.
- DequeueFront removes node from head.
- DequeueRear removes node from tail.
- Update head/tail pointers accordingly.
- Keep track of list size for empty checks.

Full Transcript

This visual execution shows how a dequeue works using a linked list. We add nodes at the tail end and remove nodes from either the front (head) or rear (tail). Each operation updates the head or tail pointers to keep the list connected. The variable tracker shows how head, tail, and list size change after each operation. Key moments clarify why pointers move as they do. The quiz tests understanding of pointer positions and list size changes during operations.

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