Queue and Stack behavior in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Queue operation?

Consider the following C# code using a Queue. What will be printed to the console?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        Queue<int> q = new Queue<int>();
        q.Enqueue(10);
        q.Enqueue(20);
        q.Enqueue(30);
        Console.WriteLine(q.Dequeue());
        Console.WriteLine(q.Peek());
    }
}

A 20\n30

B 10\n30

C 10\n20

D 20\n10

💡 Hint

Remember, Queue follows FIFO: first in, first out.

The first Enqueue adds 10, then 20, then 30. Dequeue removes and returns the first item (10). Peek returns the next item without removing it (20).

❓ Predict Output

intermediate

What is the output of this Stack operation?

Look at this C# code using a Stack. What will be printed?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        Stack<string> s = new Stack<string>();
        s.Push("apple");
        s.Push("banana");
        s.Push("cherry");
        Console.WriteLine(s.Pop());
        Console.WriteLine(s.Peek());
    }
}

A cherry\nbanana

B apple\nbanana

C banana\napple

D cherry\napple

💡 Hint

Stack follows LIFO: last in, first out.

Push adds items on top. Pop removes and returns the top item (cherry). Peek returns the new top item (banana) without removing it.

🔧 Debug

advanced

Why does this Queue code throw an exception?

This C# code throws an InvalidOperationException at runtime. Why?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        Queue<int> q = new Queue<int>();
        q.Enqueue(1);
        q.Dequeue();
        q.Dequeue();
    }
}

A Because Dequeue is called on an empty Queue after removing the only element.

B Because Enqueue was not called before Dequeue.

C Because Queue does not support Dequeue operation.

D Because the Queue was not initialized properly.

💡 Hint

Check what happens when you remove more items than exist.

After Enqueue(1), the Queue has one item. The first Dequeue removes it. The second Dequeue tries to remove from an empty Queue, causing InvalidOperationException.

❓ Predict Output

advanced

What is the final content of the Stack?

After running this C# code, what items remain in the Stack (from top to bottom)?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        Stack<int> s = new Stack<int>();
        s.Push(5);
        s.Push(10);
        s.Push(15);
        s.Pop();
        s.Push(20);
        foreach(var item in s) {
            Console.Write(item + " ");
        }
    }
}

A 5 10 20

B 20 10 5

C 10 5 20

D 15 20 10 5

💡 Hint

Remember Pop removes the top item, Push adds on top, and foreach iterates from top to bottom.

Stack after pushes: 5 (bottom), 10, 15 (top). Pop removes 15. Push 20 adds on top. Final stack top to bottom: 20, 10, 5.

🧠 Conceptual

expert

Which statement correctly describes the difference between Queue and Stack?

Choose the option that best explains the fundamental difference between Queue and Stack data structures.

A Queue and Stack both remove elements randomly without any order.

B Stack removes elements in the order they were added; Queue removes elements in reverse order of addition.

C Both Queue and Stack remove elements in the order they were added, but Stack is faster.

D Queue removes elements in the order they were added; Stack removes elements in reverse order of addition.

💡 Hint

Think about FIFO vs LIFO.

Queue follows FIFO (first in, first out), so elements are removed in the order they were added. Stack follows LIFO (last in, first out), so elements are removed in reverse order.