Enqueue Operation in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Enqueue in Circular Queue

What is the printed state of the queue after these enqueue operations on a circular queue of size 5?

Operations:
Enqueue 10, Enqueue 20, Enqueue 30, Enqueue 40, Enqueue 50

DSA C#

class CircularQueue {
    private int[] arr;
    private int front, rear, size, capacity;

    public CircularQueue(int capacity) {
        this.capacity = capacity;
        arr = new int[capacity];
        front = 0;
        rear = -1;
        size = 0;
    }

    public void Enqueue(int item) {
        if (size == capacity) {
            Console.WriteLine("Queue is full");
            return;
        }
        rear = (rear + 1) % capacity;
        arr[rear] = item;
        size++;
    }

    public string PrintQueue() {
        if (size == 0) return "Queue is empty";
        string result = "";
        for (int i = 0; i < size; i++) {
            int index = (front + i) % capacity;
            result += arr[index] + " -> ";
        }
        return result + "null";
    }
}

var q = new CircularQueue(5);
q.Enqueue(10);
q.Enqueue(20);
q.Enqueue(30);
q.Enqueue(40);
q.Enqueue(50);
Console.WriteLine(q.PrintQueue());

A Queue is full

B 50 -> 40 -> 30 -> 20 -> 10 -> null

C 10 -> 20 -> 30 -> 40 -> 50 -> null

D null

💡 Hint

Remember that enqueue adds elements at the rear and the queue prints from front to rear.

The circular queue adds elements at the rear index, wrapping around if needed. Since the queue is empty initially, elements are added in order 10, 20, 30, 40, 50. Printing from front to rear shows them in the same order.

❓ Predict Output

intermediate

Enqueue Overflow Behavior

What will be the output after trying to enqueue an element into a full queue of capacity 3?

Operations:
Enqueue 1, Enqueue 2, Enqueue 3, Enqueue 4

DSA C#

class SimpleQueue {
    private int[] arr;
    private int front, rear, size, capacity;

    public SimpleQueue(int capacity) {
        this.capacity = capacity;
        arr = new int[capacity];
        front = 0;
        rear = -1;
        size = 0;
    }

    public void Enqueue(int item) {
        if (size == capacity) {
            Console.WriteLine("Queue is full");
            return;
        }
        rear++;
        arr[rear] = item;
        size++;
    }

    public string PrintQueue() {
        if (size == 0) return "Queue is empty";
        string result = "";
        for (int i = front; i <= rear; i++) {
            result += arr[i] + " -> ";
        }
        return result + "null";
    }
}

var q = new SimpleQueue(3);
q.Enqueue(1);
q.Enqueue(2);
q.Enqueue(3);
q.Enqueue(4);
Console.WriteLine(q.PrintQueue());

A

Queue is full
1 -> 2 -> 3 -> null

B 1 -> 2 -> 3 -> 4 -> null

C

Queue is full
null

D 1 -> 2 -> null

💡 Hint

Check what happens when size equals capacity before adding a new element.

The queue has capacity 3 and already contains 3 elements. Trying to enqueue the 4th element prints "Queue is full" and does not add it. The queue remains with 1, 2, 3.

🔧 Debug

advanced

Find the Bug in Enqueue Method

This enqueue method for a queue has a bug. What error will it cause when running?

Code snippet:

public void Enqueue(int item) {
    if (size == capacity) {
        Console.WriteLine("Queue is full");
        return;
    }
    rear = (rear + 1) % capacity;
    arr[rear] = item;
    size++;
}

DSA C#

public class Queue {
    private int[] arr;
    private int front, rear, size, capacity;

    public Queue(int capacity) {
        this.capacity = capacity;
        arr = new int[capacity];
        front = 0;
        rear = -1;
        size = 0;
    }

    public void Enqueue(int item) {
        if (size == capacity) {
            Console.WriteLine("Queue is full");
            return;
        }
        rear = (rear + 1) % capacity;
        arr[rear] = item;
        size++;
    }

    public int Dequeue() {
        if (size == 0) throw new InvalidOperationException("Queue is empty");
        int item = arr[front];
        front = (front + 1) % capacity;
        size--;
        return item;
    }
}

var q = new Queue(3);
q.Enqueue(10);
q.Enqueue(20);
q.Enqueue(30);
q.Enqueue(40);

A IndexOutOfRangeException at arr[rear] assignment

B NullReferenceException

C InvalidOperationException thrown

D No error, prints "Queue is full" when full

💡 Hint

Check how rear is updated and how size is checked before enqueue.

The enqueue method correctly checks if the queue is full before adding. It updates rear with modulo to wrap around. When full, it prints "Queue is full" and returns. No exception occurs.

🧠 Conceptual

advanced

Enqueue Operation Complexity

What is the time complexity of the enqueue operation in a circular queue implemented with an array?

A O(1) - Constant time

B O(log n) - Logarithmic time

C O(n) - Linear time

D O(n^2) - Quadratic time

💡 Hint

Enqueue only updates a few variables and places one element.

Enqueue in a circular queue just moves the rear pointer and inserts the element. These are constant time operations, so complexity is O(1).

🚀 Application

expert

Resulting Queue After Mixed Operations

Given a circular queue of capacity 4, what is the printed queue after these operations?

Operations:
Enqueue 5, Enqueue 10, Enqueue 15, Dequeue, Enqueue 20, Enqueue 25

DSA C#

class CircularQueue {
    private int[] arr;
    private int front, rear, size, capacity;

    public CircularQueue(int capacity) {
        this.capacity = capacity;
        arr = new int[capacity];
        front = 0;
        rear = -1;
        size = 0;
    }

    public void Enqueue(int item) {
        if (size == capacity) {
            Console.WriteLine("Queue is full");
            return;
        }
        rear = (rear + 1) % capacity;
        arr[rear] = item;
        size++;
    }

    public int Dequeue() {
        if (size == 0) {
            Console.WriteLine("Queue is empty");
            return -1;
        }
        int item = arr[front];
        front = (front + 1) % capacity;
        size--;
        return item;
    }

    public string PrintQueue() {
        if (size == 0) return "Queue is empty";
        string result = "";
        for (int i = 0; i < size; i++) {
            int index = (front + i) % capacity;
            result += arr[index] + " -> ";
        }
        return result + "null";
    }
}

var q = new CircularQueue(4);
q.Enqueue(5);
q.Enqueue(10);
q.Enqueue(15);
q.Dequeue();
q.Enqueue(20);
q.Enqueue(25);
Console.WriteLine(q.PrintQueue());

A 5 -> 10 -> 15 -> 20 -> null

B 10 -> 15 -> 20 -> 25 -> null

C 15 -> 20 -> 25 -> null

D Queue is full

💡 Hint

Track front and rear indices carefully after each operation.

Initially enqueue 5,10,15 fills queue: front=0, rear=2, size=3.
Dequeue removes 5, front moves to 1, size=2.
Enqueue 20 at rear=3, size=3.
Enqueue 25 wraps rear to 0, size=4.
Queue elements from front=1: 10,15,20,25.

Practice

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

easy

A. Adds an element to the end of the queue

B. Removes an element from the front of the queue

C. Checks if the queue is empty

D. Returns the size of the queue

Solution

1. Step 1: Understand queue behavior

   A queue works like a line where new people join at the end.

2. Step 2: Define enqueue operation

   Enqueue means adding a new element at the back of the queue.

3. Final Answer:

   Adds an element to the end of the queue -> Option A

4. Quick Check:

   Enqueue = Add at end [OK]

Hint: Enqueue means add at the back of the queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue
• Thinking enqueue removes elements
• Mixing enqueue with checking size

2. Which of the following is the correct syntax to enqueue an integer value 10 into a Queue<int> named q in C#?

easy

A. q.Enqueue(10);

B. q.Add(10);

C. q.Push(10);

D. q.Insert(10);

Solution

1. Step 1: Recall C# Queue method

   The method to add elements in Queue<T> is Enqueue.

2. Step 2: Match syntax

   Calling q.Enqueue(10); adds 10 to the queue correctly.

3. Final Answer:

   q.Enqueue(10); -> Option A

4. Quick Check:

   Use Enqueue() to add in C# Queue [OK]

Hint: Use Enqueue() method to add elements in C# Queue [OK]

Common Mistakes:

• Using Add() which is not in Queue
• Using Push() which is for stacks
• Using Insert() which is not a Queue method

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");
Console.WriteLine(string.Join(", ", q));

medium

A. cherry, banana, apple

B. apple, banana, cherry

C. banana, cherry, apple

D. apple, cherry, banana

Solution

1. Step 1: Trace enqueue order

   Elements are added in order: apple, then banana, then cherry.

2. Step 2: Understand queue order

   Queue keeps order of insertion, so printing shows apple, banana, cherry.

3. Final Answer:

   apple, banana, cherry -> Option B

4. Quick Check:

   Queue prints in enqueue order [OK]

Hint: Queue keeps insertion order; output matches enqueue sequence [OK]

Common Mistakes:

• Assuming reverse order output
• Mixing enqueue with stack behavior
• Confusing output order with dequeue

4. Identify the error in the following code snippet that tries to enqueue an element into a queue:

Queue<int> q = new Queue<int>();
q.Enqueue(5);
q.Enqueue(10);
q.Enqueue(15);
q.Enqueue();

medium

A. Cannot enqueue integer values

B. Queue is not initialized

C. Enqueue method does not exist

D. Missing argument in the last Enqueue call

Solution

1. Step 1: Check method calls

   The last call q.Enqueue(); has no argument, but Enqueue requires one.

2. Step 2: Confirm method signature

   Enqueue needs a value to add, so missing argument causes error.

3. Final Answer:

   Missing argument in the last Enqueue call -> Option D

4. Quick Check:

   Enqueue requires one argument [OK]

Hint: Enqueue always needs one argument to add [OK]

Common Mistakes:

• Calling Enqueue without argument
• Assuming Enqueue can be empty
• Confusing initialization errors

5. You have a queue q with elements [1, 2, 3]. You want to enqueue elements 4 and 5, but only if the queue size is less than 5. Which code snippet correctly performs this conditional enqueue operation in C#?

hard

A. if (q.Count == 5) { q.Enqueue(4); q.Enqueue(5); }

B. if (q.Count <= 5) { q.Enqueue(4); q.Enqueue(5); }

C. if (q.Count < 5) { q.Enqueue(4); q.Enqueue(5); }

D. if (q.Count > 5) { q.Enqueue(4); q.Enqueue(5); }

Solution

1. Step 1: Understand queue size condition

   We want to add only if size is less than 5, so condition is q.Count < 5.

2. Step 2: Apply enqueue calls inside condition

   If condition is true, enqueue 4 and 5 to add them safely.

3. Final Answer:

   if (q.Count < 5) { q.Enqueue(4); q.Enqueue(5); } -> Option C

4. Quick Check:

   Use < 5 to check size before enqueue [OK]

Hint: Check queue size with Count before enqueueing [OK]

Common Mistakes:

• Using <= instead of < causing overflow
• Using > or == which are wrong conditions
• Not checking size before enqueue