Stack Concept and LIFO Principle in DSA C# - Build from Scratch

Stack Concept and LIFO Principle

📖 Scenario: Imagine you have a stack of books on your desk. You can only add or remove the top book. This is called Last In, First Out (LIFO). We will create a stack of numbers and see how this works in code.

🎯 Goal: Build a simple stack using a list in C# and demonstrate the LIFO principle by adding and removing items.

📋 What You'll Learn

Create a list called stack with initial numbers

Create a variable called newItem to add to the stack

Add newItem to the top of the stack

Remove the top item from the stack

Print the final state of the stack showing all items separated by ' -> ' and ending with 'null'

💡 Why This Matters

🌍 Real World

Stacks are used in undo features in apps, browser history, and managing function calls in programs.

💼 Career

Understanding stacks helps in software development, debugging, and working with algorithms that require backtracking or order reversal.

1

Create the initial stack

Create a list of integers called stack with these exact values in order: 10, 20, 30

DSA C#

# Create a list called stack with 10, 20, 30
// Your code here

Hint

Use List<int> and initialize it with { 10, 20, 30 }.

2

Add a new item to the stack

Create an integer variable called newItem and set it to 40

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List stack = new List { 10, 20, 30 };
        // Create newItem and set to 40
        // Your code here
    }
}

Hint

Use int newItem = 40; to create the variable.

3

Push and pop items on the stack

Add newItem to the end of stack using stack.Add(newItem), then remove the last item from stack using stack.RemoveAt(stack.Count - 1)

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List stack = new List { 10, 20, 30 };
        int newItem = 40;
        // Add newItem to stack
        // Remove the last item from stack
        // Your code here
    }
}

Hint

Use stack.Add(newItem); to push and stack.RemoveAt(stack.Count - 1); to pop.

4

Print the final stack

Print the contents of stack separated by " -> " and ending with "null" using string.Join(" -> ", stack) inside Console.WriteLine

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List stack = new List { 10, 20, 30 };
        int newItem = 40;
        stack.Add(newItem);
        stack.RemoveAt(stack.Count - 1);
        // Print stack with ' -> ' separator and ending with 'null'
        // Your code here
    }
}

Hint

Use Console.WriteLine(string.Join(" -> ", stack) + " -> null"); to print.

Practice

1. What does the LIFO principle mean in the context of a stack?

easy

A. Last In, First Out - the first element added is the first to be removed

B. First In, First Out - the first element added is the first to be removed

C. Last In, First Out - the last element added is the first to be removed

D. First In, Last Out - the first element added is the last to be removed

Solution

1. Step 1: Understand LIFO meaning

   LIFO means the last item you put in is the first one you take out.

2. Step 2: Apply to stack behavior

   Stacks follow LIFO, so the last pushed element is popped first.

3. Final Answer:

   Last In, First Out - the last element added is the first to be removed -> Option C

4. Quick Check:

   LIFO = Last In, First Out [OK]

Hint: Remember stack is like a stack of plates, last added removed first [OK]

Common Mistakes:

• Confusing LIFO with FIFO
• Thinking first added is removed first
• Mixing up the order of insertion and removal

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

easy

A. myStack.Enqueue(10);

B. myStack.Push(10);

C. myStack.Insert(10);

D. myStack.Add(10);

Solution

1. Step 1: Recall stack push method

   In C#, the method to add an element to a stack is Push.

2. Step 2: Check method names

   Add and Insert are not stack methods; Enqueue is for queues.

3. Final Answer:

   myStack.Push(10); -> Option B

4. Quick Check:

   Push() adds element to stack [OK]

Hint: Use Push() to add elements to stack in C# [OK]

Common Mistakes:

• Using Add() or Insert() which are not stack methods
• Confusing stack with queue methods like Enqueue()
• Forgetting parentheses in method call

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

Stack<int> stack = new Stack<int>();
stack.Push(5);
stack.Push(10);
stack.Push(15);
Console.WriteLine(stack.Pop());
Console.WriteLine(stack.Peek());

medium

A. 15
10

B. 5
10

C. 10
15

D. 15
15

Solution

1. Step 1: Trace Push operations

   Elements pushed in order: 5 (bottom), 10, 15 (top).

2. Step 2: Trace Pop and Peek

   Pop removes and returns 15 (top). Peek returns new top 10 without removing.

3. Final Answer:

   15\n10 -> Option A

4. Quick Check:

   Pop() = 15, Peek() = 10 [OK]

Hint: Pop returns top and removes it; Peek returns top without removing [OK]

Common Mistakes:

• Mixing Pop and Peek outputs
• Assuming Pop does not remove element
• Confusing order of pushed elements

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

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

medium

A. Stack must be initialized with capacity

B. Pop() cannot be assigned to a string variable

C. Push() method requires two arguments

D. Stack does not have a method named Top()

Solution

1. Step 1: Check stack methods

   Stack in C# has Peek() to view top element, not Top().

2. Step 2: Validate other statements

   Pop() returns string, Push() takes one argument, no capacity needed.

3. Final Answer:

   Stack does not have a method named Top() -> Option D

4. Quick Check:

   Use Peek() to view top element [OK]

Hint: Use Peek() to see top element, not Top() [OK]

Common Mistakes:

• Using Top() instead of Peek()
• Thinking Pop() returns void
• Assuming Push() needs capacity argument

5. Given a stack with elements pushed in this order: 1, 2, 3, 4, 5. You pop two elements, then push 6 and 7. What is the order of elements if you pop all remaining elements one by one?

hard

A. 7, 6, 3, 2, 1

B. 3, 2, 6, 7

C. 5, 4, 6, 7

D. 7, 6, 3, 2

Solution

1. Step 1: Track initial pushes

   Stack after pushes: bottom 1,2,3,4,5 top.

2. Step 2: Pop two elements

   Pop removes 5 then 4; stack now 1,2,3 (bottom to top).

3. Step 3: Push 6 and 7

   Stack now: 1,2,3,6,7 (bottom to top).

4. Step 4: Pop all remaining elements

   Pop order (first popped to last): 7,6,3,2,1.

5. Final Answer:

   7, 6, 3, 2, 1 -> Option A

6. Quick Check:

   Pop order after operations = 7,6,3,2,1 [OK]

Hint: Pop removes last pushed; track pushes and pops stepwise [OK]

Common Mistakes:

• Forgetting to remove popped elements
• Mixing push and pop order
• Ignoring elements still in stack after operations