Dequeue Operation in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Dequeue Operations on a Double-Ended Queue

What is the printed state of the deque after performing the following operations in C#?

Start with an empty deque.
Enqueue 10 at the rear.
Enqueue 20 at the rear.
Enqueue 5 at the front.
Dequeue from the rear.
Dequeue from the front.
Print the deque.

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        LinkedList<int> deque = new LinkedList<int>();
        deque.AddLast(10); // rear
        deque.AddLast(20); // rear
        deque.AddFirst(5); // front
        deque.RemoveLast(); // dequeue rear
        deque.RemoveFirst(); // dequeue front
        foreach (var item in deque) {
            Console.Write(item + " -> ");
        }
        Console.WriteLine("null");
    }
}

A 10 -> null

B 5 -> 10 -> null

C 5 -> 20 -> null

D 20 -> null

💡 Hint

Remember that AddFirst adds to front and AddLast adds to rear. RemoveFirst removes from front and RemoveLast removes from rear.

Initially, deque is empty. After AddLast(10) and AddLast(20), deque is 10 -> 20. AddFirst(5) makes it 5 -> 10 -> 20. RemoveLast() removes 20, so deque is 5 -> 10. RemoveFirst() removes 5, leaving 10 only. So the output is '10 -> null'.

❓ Predict Output

intermediate

Resulting Deque After Mixed Operations

Given the following C# code, what is the final printed deque state?

Start empty.
Enqueue 1 at front.
Enqueue 2 at rear.
Enqueue 3 at front.
Dequeue from rear.
Print the deque.

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        LinkedList<int> deque = new LinkedList<int>();
        deque.AddFirst(1);
        deque.AddLast(2);
        deque.AddFirst(3);
        deque.RemoveLast();
        foreach (var item in deque) {
            Console.Write(item + " -> ");
        }
        Console.WriteLine("null");
    }
}

A 3 -> 1 -> 2 -> null

B 3 -> 1 -> null

C 1 -> 3 -> null

D 1 -> 2 -> null

💡 Hint

AddFirst adds to front, AddLast adds to rear, RemoveLast removes from rear.

After AddFirst(1), deque is 1. AddLast(2) makes it 1 -> 2. AddFirst(3) makes it 3 -> 1 -> 2. RemoveLast() removes 2, leaving 3 -> 1. So output is '3 -> 1 -> null'.

🔧 Debug

advanced

Identify the Error in Dequeue Operation Code

What error will this C# code produce when run?

Code:
LinkedList deque = new LinkedList();
deque.AddLast(1);
deque.RemoveFirst();
deque.RemoveFirst();

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        LinkedList<int> deque = new LinkedList<int>();
        deque.AddLast(1);
        deque.RemoveFirst();
        deque.RemoveFirst();
    }
}

A IndexOutOfRangeException

B NullReferenceException

C No error, runs fine

D InvalidOperationException

💡 Hint

Removing from an empty LinkedList causes an exception.

After adding 1, RemoveFirst() removes it, deque becomes empty. The next RemoveFirst() tries to remove from empty deque, causing InvalidOperationException.

❓ Predict Output

advanced

Deque State After Multiple Front and Rear Operations

What is the output of the following C# code?

Start empty.
AddLast(4), AddFirst(3), AddLast(5), RemoveFirst(), AddFirst(2), RemoveLast(), Print deque.

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        LinkedList<int> deque = new LinkedList<int>();
        deque.AddLast(4);    // 4
        deque.AddFirst(3);   // 3 -> 4
        deque.AddLast(5);    // 3 -> 4 -> 5
        deque.RemoveFirst(); // removes 3, deque: 4 -> 5
        deque.AddFirst(2);   // 2 -> 4 -> 5
        deque.RemoveLast();  // removes 5, deque: 2 -> 4
        foreach (var item in deque) {
            Console.Write(item + " -> ");
        }
        Console.WriteLine("null");
    }
}

A 2 -> 4 -> null

B 3 -> 4 -> null

C 4 -> 5 -> null

D 2 -> 5 -> null

💡 Hint

Track each operation carefully, front and rear add/remove.

After all operations, deque contains 2 at front and 4 at rear, so output is '2 -> 4 -> null'.

🧠 Conceptual

expert

Number of Items in Deque After Sequence of Operations

A deque starts empty. The following operations are performed:
1. AddFirst(10)
2. AddLast(20)
3. RemoveFirst()
4. AddLast(30)
5. RemoveLast()
6. AddFirst(40)
How many items are in the deque at the end?

A 3

B 1

C 2

D 0

💡 Hint

Count items after each operation carefully.

Step by step:
Empty
AddFirst(10): 10
AddLast(20): 10, 20
RemoveFirst(): removes 10, left 20
AddLast(30): 20, 30
RemoveLast(): removes 30, left 20
AddFirst(40): 40, 20
So 2 items remain.

Practice

1. What does the Dequeue operation do in a queue data structure?

easy

A. Returns the number of elements in the queue

B. Adds an element to the end of the queue

C. Checks if the queue is empty

D. Removes and returns the element at the front of the queue

Solution

1. Step 1: Understand the queue structure

   A queue works like a line where the first person in line is served first (FIFO).

2. Step 2: Define the Dequeue operation

   Dequeue removes and returns the first element in the queue, following FIFO.

3. Final Answer:

   Removes and returns the element at the front of the queue -> Option D

4. Quick Check:

   Dequeue = Remove front element [OK]

Hint: Dequeue always removes from the front [OK]

Common Mistakes:

• Confusing Dequeue with Enqueue (adding element)
• Thinking Dequeue checks queue size
• Assuming Dequeue removes from the end

2. Which of the following is the correct syntax to remove an element from a Queue<int> named q in C#?

easy

A. q.Remove()

B. q.Pop()

C. q.Dequeue()

D. q.Delete()

Solution

1. Step 1: Recall C# Queue methods

   The method to remove and return the front element in C# Queue is Dequeue().

2. Step 2: Check each option

   Only q.Dequeue() matches the correct method name and syntax.

3. Final Answer:

   q.Dequeue() -> Option C

4. Quick Check:

   Queue removal method = Dequeue() [OK]

Hint: Use Dequeue() to remove front element in C# Queue [OK]

Common Mistakes:

• Using Remove() which does not exist for Queue
• Using Pop() which is for stacks
• Using Delete() which is invalid

3. What will be the output of the following C# code?

Queue<string> q = new Queue<string>();
q.Enqueue("apple");
q.Enqueue("banana");
q.Enqueue("cherry");
string first = q.Dequeue();
Console.WriteLine(first);
Console.WriteLine(q.Peek());

medium

A. apple\nbanana

B. banana\ncherry

C. apple\ncherry

D. cherry\napple

Solution

1. Step 1: Trace Enqueue operations

   Queue after enqueues: front -> apple, banana, cherry -> rear.

2. Step 2: Dequeue removes 'apple'

   Dequeue returns 'apple' and removes it from queue.

3. Step 3: Peek returns new front 'banana'

   After dequeue, front is 'banana', so Peek returns 'banana'.

4. Final Answer:

   apple banana -> Option A

5. Quick Check:

   Dequeue = apple, Peek = banana [OK]

Hint: Dequeue returns front; Peek shows new front [OK]

Common Mistakes:

• Confusing Peek with Dequeue
• Assuming Dequeue removes from rear
• Mixing order of elements

4. Identify the error in the following C# code that uses a queue:

Queue<int> q = new Queue<int>();
q.Enqueue(10);
q.Enqueue(20);
int x = q.Dequeue();
int y = q.Dequeue();
int z = q.Dequeue(); // Error here
Console.WriteLine(z);

medium

A. Queue does not support Dequeue method

B. Dequeue called on empty queue causes InvalidOperationException

C. Enqueue method is used incorrectly

D. Console.WriteLine cannot print integers

Solution

1. Step 1: Count elements and Dequeue calls

   Queue has 2 elements; 3 Dequeue calls are made.

2. Step 2: Third Dequeue on empty queue causes error

   Calling Dequeue on empty queue throws InvalidOperationException.

3. Final Answer:

   Dequeue called on empty queue causes InvalidOperationException -> Option B

4. Quick Check:

   Empty queue Dequeue = Exception [OK]

Hint: Dequeue on empty queue throws exception [OK]

Common Mistakes:

• Assuming Dequeue returns default value on empty
• Thinking Enqueue is incorrect
• Believing Console.WriteLine can't print int

5. You have a queue with elements [5, 10, 15, 20] (front to rear). You perform these operations:
1. Dequeue
2. Enqueue(25)
3. Dequeue
4. Enqueue(30)
What is the final state of the queue (front to rear)?

hard

A. [15, 20, 25, 30]

B. [10, 15, 20, 25]

C. [20, 25, 30]

D. [5, 10, 15, 20, 25, 30]

Solution

1. Step 1: Initial queue and first Dequeue

   Queue: 5(front),10,15,20(rear). Dequeue removes 5. Queue now: 10,15,20.

2. Step 2: Enqueue 25

   Add 25 at rear. Queue: 10,15,20,25.

3. Step 3: Second Dequeue

   Remove front 10. Queue: 15,20,25.

4. Step 4: Enqueue 30

   Add 30 at rear. Queue: 15,20,25,30.

5. Final Answer:

   [15, 20, 25, 30] -> Option A

6. Quick Check:

   After ops queue front to rear = [15,20,25,30] [OK]

Hint: Track front and rear after each operation [OK]

Common Mistakes:

• Forgetting to remove elements on Dequeue
• Adding new elements at front instead of rear
• Mixing order of elements after operations