Pop Operation on Stack in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after popping from the stack?

Consider the following C# code that uses a stack. What will be printed after the pop operation?

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        Stack<int> stack = new Stack<int>();
        stack.Push(10);
        stack.Push(20);
        stack.Push(30);
        int popped = stack.Pop();
        Console.WriteLine(popped);
    }
}

A 10

B 30

C 20

D Throws InvalidOperationException

💡 Hint

Remember, stack is Last In First Out (LIFO). The last pushed item is popped first.

The stack has elements pushed in order: 10, then 20, then 30. The pop operation removes and returns the last pushed element, which is 30.

❓ Predict Output

intermediate

What is the stack content after popping?

Given the following C# code, what is the content of the stack after the pop operation? Show the stack from top to bottom.

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        Stack<string> stack = new Stack<string>();
        stack.Push("apple");
        stack.Push("banana");
        stack.Push("cherry");
        stack.Pop();
        foreach (var item in stack) {
            Console.Write(item + " -> ");
        }
        Console.WriteLine("null");
    }
}

A banana -> apple -> null

B apple -> banana -> null

C cherry -> banana -> apple -> null

D apple -> cherry -> null

💡 Hint

Pop removes the top element. The stack is LIFO, so the last pushed is on top.

Initially, the stack is cherry (top), banana, apple (bottom). After popping cherry, banana becomes the top, followed by apple.

🧠 Conceptual

advanced

What happens if you pop from an empty stack?

In C#, what exception is thrown when you call Pop() on an empty Stack?

A IndexOutOfRangeException

B NullReferenceException

C InvalidOperationException

D ArgumentException

💡 Hint

Think about what happens when you try to remove an element that does not exist.

Pop on an empty stack throws InvalidOperationException because the operation is invalid when the stack has no elements.

❓ Predict Output

advanced

What is the output after multiple pop operations?

What will be printed after executing the following C# code?

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        Stack<int> stack = new Stack<int>();
        stack.Push(1);
        stack.Push(2);
        stack.Push(3);
        Console.WriteLine(stack.Pop());
        Console.WriteLine(stack.Pop());
        Console.WriteLine(stack.Peek());
    }
}

A

3
2
1

B

1
2
3

C

3
2
2

D

3
1
2

💡 Hint

Pop removes and returns the top element. Peek returns the top element without removing it.

Stack after pushes: top=3, then 2, then 1. First pop returns 3, second pop returns 2, then peek returns 1 (top now).

🔧 Debug

expert

Identify the error in pop usage

What error will this C# code produce when executed?

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        Stack<int> stack = new Stack<int>();
        int x = stack.Pop();
        Console.WriteLine(x);
    }
}

A NullReferenceException

B Compilation error

C No error, prints 0

D InvalidOperationException

💡 Hint

What happens if you pop from an empty stack?

Pop on an empty stack throws InvalidOperationException at runtime.

Practice

1. What does the Pop() operation do in a stack?

easy

A. Adds an element to the bottom of the stack

B. Clears all elements from the stack

C. Returns the bottom element without removing it

D. Removes and returns the top element of the stack

Solution

1. Step 1: Understand the stack behavior

   A stack follows Last In First Out (LIFO) principle, so the last added element is on top.

2. Step 2: Define the Pop operation

   Pop removes and returns the top element, which is the last inserted item.

3. Final Answer:

   Removes and returns the top element of the stack -> Option D

4. Quick Check:

   Pop = remove top element [OK]

Hint: Pop always removes the last added item [OK]

Common Mistakes:

• Thinking Pop adds elements
• Confusing Pop with Peek
• Assuming Pop removes bottom element

2. Which of the following is the correct syntax to pop an element from a stack named myStack in C#?

easy

A. myStack.Pop();

B. myStack.pop();

C. Pop(myStack);

D. myStack.PopElement();

Solution

1. Step 1: Recall C# method naming conventions

   In C#, method names are case-sensitive and usually PascalCase.

2. Step 2: Identify the correct method call

   The Stack class has a method named Pop() with uppercase P, called on the stack instance.

3. Final Answer:

   myStack.Pop(); -> Option A

4. Quick Check:

   Correct method name and syntax = myStack.Pop(); [OK]

Hint: C# methods are case-sensitive and PascalCase [OK]

Common Mistakes:

• Using lowercase 'pop()' instead of 'Pop()'
• Trying to call Pop as a standalone function
• Using incorrect method names like PopElement

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

Stack<int> stack = new Stack<int>();
stack.Push(10);
stack.Push(20);
stack.Push(30);
int popped = stack.Pop();
Console.WriteLine(popped);
Console.WriteLine(string.Join(", ", stack));

medium

A. 10
20, 30

B. 30
20, 10

C. 20
10, 30

D. 30
10, 20

Solution

1. Step 1: Trace the Push operations

   Elements pushed in order: 10 (bottom), 20, 30 (top).

2. Step 2: Pop removes top element

   Pop removes 30, so popped = 30. Remaining stack: 20 (top), 10 (bottom).

3. Step 3: Print statements

   First line prints popped = 30. Second line prints stack elements from top to bottom: 20, 10.

4. Final Answer:

   30 20, 10 -> Option B

5. Quick Check:

   Pop returns top (30), stack now 20, 10 [OK]

Hint: Pop returns last pushed; stack order reverses on print [OK]

Common Mistakes:

• Confusing order of elements after pop
• Printing stack in wrong order
• Assuming Pop returns bottom element

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

Stack<string> stack = new Stack<string>();
stack.Push("apple");
stack.Push("banana");
string fruit = stack.Pop();
Console.WriteLine(fruit);
Console.WriteLine(stack.Pop());
Console.WriteLine(stack.Pop());

medium

A. Pop method is called incorrectly

B. No error, code runs fine

C. Third Pop causes InvalidOperationException (stack empty)

D. Stack should be initialized with capacity

Solution

1. Step 1: Count Push and Pop calls

   Two Push calls add "apple" and "banana". Three Pop calls remove elements.

2. Step 2: Analyze stack state after Pops

   First Pop removes "banana", second Pop removes "apple", third Pop tries to remove from empty stack causing error.

3. Final Answer:

   Third Pop causes InvalidOperationException (stack empty) -> Option C

4. Quick Check:

   Pop on empty stack throws exception [OK]

Hint: Pop more times than Push causes error [OK]

Common Mistakes:

• Ignoring stack empty error
• Assuming Pop returns null if empty
• Thinking stack needs capacity on init

5. Given a stack with elements [5, 10, 15, 20] (5 at bottom, 20 at top), what will be the stack content after performing two Pop() operations followed by one Push(25) operation?

hard

A. [5, 10, 25]

B. [5, 10, 15, 25]

C. [10, 15, 25]

D. [5, 15, 25]

Solution

1. Step 1: Understand initial stack order

   Stack bottom to top: 5, 10, 15, 20.

2. Step 2: Perform two Pop operations

   First Pop removes 20, second Pop removes 15. Remaining stack: 5, 10.

3. Step 3: Perform Push(25)

   Push adds 25 on top. Final stack: 5, 10, 25.

4. Final Answer:

   [5, 10, 25] -> Option A

5. Quick Check:

   Pop twice removes top two; Push adds new top [OK]

Hint: Pop removes top; Push adds new top [OK]

Common Mistakes:

• Removing wrong elements on Pop
• Adding Push element at bottom
• Confusing stack order after operations