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

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after pushing elements on the stack?

Consider the following C# code that pushes elements onto a stack. What will be the printed output?

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);

        foreach (int item in stack) {
            Console.Write(item + " -> ");
        }
        Console.WriteLine("null");
    }
}

A 10 -> 20 -> 30 -> null

B 30 -> 20 -> 10 -> null

C null -> 10 -> 20 -> 30

D 20 -> 30 -> 10 -> null

💡 Hint

Remember that Stack is Last-In-First-Out (LIFO). The last pushed element is on top.

The stack pushes 10, then 20, then 30. The top is 30, so iterating prints 30 first, then 20, then 10.

❓ Predict Output

intermediate

What is the stack content after pushing and popping?

What will be the output after executing the following code?

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 (string fruit in stack) {
            Console.Write(fruit + " -> ");
        }
        Console.WriteLine("null");
    }
}

A cherry -> banana -> apple -> null

B apple -> banana -> null

C banana -> apple -> null

D apple -> null

💡 Hint

Pop removes the top element. Which element was last pushed?

After pushing apple, banana, cherry, the stack top is cherry. Pop removes cherry. Remaining stack is banana (top), then apple.

❓ Predict Output

advanced

What error occurs when popping from an empty stack?

What error will this code produce when executed?

DSA C#

using System;
using System.Collections.Generic;

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

A InvalidOperationException

B NullReferenceException

C IndexOutOfRangeException

D No error, returns 0

💡 Hint

Popping from empty stack is invalid in C#.

Calling Pop on an empty Stack throws InvalidOperationException in C#.

❓ Predict Output

advanced

What is the output after pushing elements and converting to array?

What will be printed by this code?

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        Stack<char> stack = new Stack<char>();
        stack.Push('a');
        stack.Push('b');
        stack.Push('c');

        char[] arr = stack.ToArray();
        foreach (char ch in arr) {
            Console.Write(ch + " ");
        }
    }
}

A c b a

B a c b

C a b c

D b c a

💡 Hint

Stack.ToArray returns elements from top to bottom.

Stack has 'c' on top, then 'b', then 'a'. ToArray returns array starting from top element.

🧠 Conceptual

expert

What is the time complexity of pushing n elements on a stack?

If you push n elements one by one onto an empty stack, what is the overall time complexity?

A O(log n)

B O(1)

C O(n^2)

D O(n)

💡 Hint

Each push is constant time. Consider total time for n pushes.

Each push operation takes O(1) time. Doing n pushes takes O(n) time overall.

Practice

1. What does the Push operation do in a stack?

easy

A. Adds an element to the top of the stack

B. Removes the top element from the stack

C. Returns the bottom element of the stack

D. Checks if the stack is empty

Solution

1. Step 1: Understand stack behavior

   A stack follows Last In First Out (LIFO) principle, where elements are added or removed from the top.

2. Step 2: Define Push operation

   The Push operation adds a new element on top of the stack, increasing its size by one.

3. Final Answer:

   Adds an element to the top of the stack -> Option A

4. Quick Check:

   Push = Add top element [OK]

Hint: Push means add on top of stack [OK]

Common Mistakes:

• Confusing Push with Pop operation
• Thinking Push removes elements
• Assuming Push accesses bottom element

2. Which of the following is the correct syntax to push an integer value 10 onto a stack named stack in C#?

easy

A. stack.Insert(10);

B. stack.Add(10);

C. stack.Push(10);

D. stack.Push = 10;

Solution

1. Step 1: Recall Stack method for adding elements

   In C#, the Stack class uses the method Push() to add elements.

2. Step 2: Check syntax correctness

   The correct syntax is calling Push with the value inside parentheses: stack.Push(10);

3. Final Answer:

   stack.Push(10); -> Option C

4. Quick Check:

   Push method call syntax = stack.Push(value) [OK]

Hint: Use stack.Push(value) to add elements [OK]

Common Mistakes:

• Using Add or Insert which are not stack methods
• Assigning value directly to Push
• Missing parentheses in method call

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

Stack<int> stack = new Stack<int>();
stack.Push(5);
stack.Push(10);
stack.Push(15);
Console.WriteLine(string.Join(", ", stack));

medium

A. 5, 10, 15

B. 15, 10, 5

C. 10, 15, 5

D. 15, 5, 10

Solution

1. Step 1: Understand push order and stack iteration

   Elements pushed are 5, then 10, then 15. Stack stores last pushed on top.

2. Step 2: Determine iteration order when printing

   Stack enumerates from top to bottom, so output order is 15, 10, 5.

3. Final Answer:

   15, 10, 5 -> Option B

4. Quick Check:

   Stack top to bottom = 15, 10, 5 [OK]

Hint: Stack prints top element first [OK]

Common Mistakes:

• Assuming stack prints in push order
• Confusing stack with queue order
• Forgetting stack is LIFO

4. Identify the error in the following C# code that tries to push elements onto a stack:

Stack<string> stack = new Stack<string>();
stack.Push "hello";
stack.Push("world");

medium

A. Push method does not exist

B. Stack cannot hold strings

C. Missing semicolon after second Push

D. Missing parentheses in first Push call

Solution

1. Step 1: Check syntax of Push calls

   The first Push call is written as stack.Push "hello"; missing parentheses.

2. Step 2: Confirm correct method usage

   Push requires parentheses around argument: stack.Push("hello");

3. Final Answer:

   Missing parentheses in first Push call -> Option D

4. Quick Check:

   Method calls need parentheses [OK]

Hint: Always use parentheses with method calls [OK]

Common Mistakes:

• Ignoring parentheses syntax
• Thinking stack can't hold strings
• Assuming missing semicolon causes error here

5. Given a stack initially empty, what will be the content of the stack after executing the following code?

Stack<int> stack = new Stack<int>();
for (int i = 1; i <= 3; i++)
{
    stack.Push(i * 2);
}
stack.Push(10);
stack.Push(stack.Pop() + 5);

hard

A. 15, 6, 4, 2

B. 10, 8, 4, 2

C. 15, 10, 6, 4, 2

D. 10, 15, 6, 4, 2

Solution

1. Step 1: Trace the for loop pushing values

   Loop pushes 2, 4, 6 in order. Stack top is 6, bottom is 2.

2. Step 2: Push 10, then pop top and push pop+5

   Push 10: stack top to bottom 10, 6, 4, 2. Pop removes 10, 10 + 5 = 15, push 15: top to bottom 15, 6, 4, 2.

3. Final Answer:

   15, 6, 4, 2 -> Option A

4. Quick Check:

   Stack after operations = 15, 6, 4, 2 [OK]

Hint: Track push/pop stepwise to find final stack [OK]

Common Mistakes:

• Confusing pop value used for new push
• Forgetting pop removes top element
• Assuming push adds without removing