Stack Implementation Using Linked List in DSA C# - Build from Scratch

Stack Implementation Using Linked List

📖 Scenario: Imagine you are building a simple undo feature for a text editor. Each action the user performs is saved on a stack. You will create a stack using a linked list to store these actions.

🎯 Goal: Build a stack data structure using a linked list in C#. You will create nodes, push actions onto the stack, pop actions off, and finally display the current stack.

📋 What You'll Learn

Create a Node class with a string data field and a Node next field

Create a Stack class with a Node top field

Implement Push method to add a node to the top

Implement Pop method to remove the top node

Print the stack from top to bottom

💡 Why This Matters

🌍 Real World

Stacks are used in undo features, expression evaluation, and backtracking algorithms.

💼 Career

Understanding stack implementation helps in software development roles involving data structures, algorithms, and system design.

1

Create Node class and Stack class with top

Create a class called Node with a public string field data and a public Node field next. Then create a class called Stack with a public Node field called top initialized to null.

DSA C#

// Define Node class and Stack class with top field
// Your code here

Hint

Think of Node as a box holding the action and a pointer to the next box. The Stack keeps track of the top box.

2

Add Push method to Stack

Inside the Stack class, add a public method called Push that takes a string parameter action. This method should create a new Node with action as data, set its next to the current top, and then update top to this new node.

DSA C#

public class Node
{
    public string data;
    public Node next;

    public Node(string data)
    {
        this.data = data;
        this.next = null;
    }
}

public class Stack
{
    public Node top = null;

    // Add Push method here
    // Your code here
}

Hint

Push adds a new action on top by linking the new node to the current top and then updating top.

3

Add Pop method to Stack

Inside the Stack class, add a public method called Pop that returns a string. This method should check if top is null and return null if so. Otherwise, it should save the data from top, update top to top.next, and return the saved data.

DSA C#

public class Node
{
    public string data;
    public Node next;

    public Node(string data)
    {
        this.data = data;
        this.next = null;
    }
}

public class Stack
{
    public Node top = null;

    public void Push(string action)
    {
        Node newNode = new Node(action);
        newNode.next = top;
        top = newNode;
    }

    // Add Pop method here
    // Your code here
}

Hint

Pop removes the top action and returns its data. If the stack is empty, return null.

4

Print the stack from top to bottom

Create a PrintStack method inside the Stack class that prints each node's data from top to the bottom, separated by -> , ending with null. Then, in Main, create a Stack object, push these actions in order: "Type A", "Type B", "Type C", pop one action, and finally call PrintStack.

DSA C#

public class Node
{
    public string data;
    public Node next;

    public Node(string data)
    {
        this.data = data;
        this.next = null;
    }
}

public class Stack
{
    public Node top = null;

    public void Push(string action)
    {
        Node newNode = new Node(action);
        newNode.next = top;
        top = newNode;
    }

    public string Pop()
    {
        if (top == null)
        {
            return null;
        }
        string poppedData = top.data;
        top = top.next;
        return poppedData;
    }

    // Add PrintStack method here
    // Your code here
}

class Program
{
    static void Main()
    {
        Stack stack = new Stack();
        // Push and Pop actions here
        // Your code here
    }
}

Hint

PrintStack walks through the linked list from top to bottom, printing each action followed by an arrow, ending with null.

In Main, push three actions, pop one, then print the stack.

Practice

1. What is the main advantage of implementing a stack using a linked list instead of an array?

easy

A. It uses less memory than an array

B. The stack can grow dynamically without a fixed size limit

C. It allows random access to elements

D. It automatically sorts the elements

Solution

1. Step 1: Understand stack size limitation in arrays

   Arrays have a fixed size, so stacks implemented with arrays have a maximum capacity.

2. Step 2: Linked list dynamic size

   Linked lists can grow or shrink dynamically by adding or removing nodes, so the stack size is limited only by memory.

3. Final Answer:

   The stack can grow dynamically without a fixed size limit -> Option B

4. Quick Check:

   Linked list stacks grow dynamically [OK]

Hint: Linked list stacks grow dynamically, arrays do not [OK]

Common Mistakes:

• Thinking linked list uses less memory always
• Assuming linked list allows random access
• Confusing stack sorting with implementation

2. Which of the following is the correct way to push an element onto a stack implemented using a linked list in C#?

easy

A. Create a new node, set its next to current top, then update top to new node

B. Create a new node, set its previous to current top, then update top to new node

C. Add the new element at the end of the linked list

D. Replace the top node's value with the new element

Solution

1. Step 1: Understand push operation in linked list stack

   Push adds a new node at the beginning (top) of the linked list.

2. Step 2: Correct pointer update

   The new node's next points to the current top, then top is updated to new node.

3. Final Answer:

   Create a new node, set its next to current top, then update top to new node -> Option A

4. Quick Check:

   Push adds node at front [OK]

Hint: Push adds node at front, update top pointer [OK]

Common Mistakes:

• Using previous pointer instead of next
• Adding node at end instead of front
• Overwriting top node value instead of adding new node

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

StackNode top = null;
// Push 10
StackNode node1 = new StackNode(10);
node1.next = top;
top = node1;
// Push 20
StackNode node2 = new StackNode(20);
node2.next = top;
top = node2;
// Pop
int popped = top.data;
top = top.next;
// Print top data
Console.WriteLine(top.data);

medium

A. NullReferenceException

B. 20

C. 10

D. 0

Solution

1. Step 1: Trace push operations

   Push 10: top points to node with data 10.
   Push 20: top points to node with data 20, next points to node 10.

2. Step 2: Trace pop operation

   Pop removes top node (20), top updated to next node (10).

3. Step 3: Print top data

   Top now points to node with data 10, so output is 10.

4. Final Answer:

   10 -> Option C

5. Quick Check:

   After pop, top data = 10 [OK]

Hint: Pop removes top, next node becomes new top [OK]

Common Mistakes:

• Printing popped value instead of new top
• Confusing push order
• Not updating top after pop

4. Identify the error in this pop method for a stack implemented using a linked list:

public int Pop() {
    if (top == null) {
        Console.WriteLine("Stack Underflow");
    }
    int value = top.data;
    top = top.next;
    return value;
}

medium

A. It does not return a value when stack is empty

B. It does not update the top pointer

C. It incorrectly prints "Stack Overflow" instead of "Underflow"

D. It removes the bottom node instead of the top

Solution

1. Step 1: Check behavior when stack is empty

   If top is null, method prints message but continues to access top.data causing error or returns no value.

2. Step 2: Missing return for empty stack case

   Method should return or throw exception when stack is empty to avoid runtime error.

3. Final Answer:

   It does not return a value when stack is empty -> Option A

4. Quick Check:

   Empty stack must return or throw [OK]

Hint: Always return or throw on empty stack pop [OK]

Common Mistakes:

• Ignoring empty stack case
• Confusing overflow with underflow
• Not updating top pointer

5. You want to implement a method ReverseStack that reverses the elements of a stack implemented using a linked list without using extra data structures. Which approach is correct?

hard

A. Use a queue to dequeue and enqueue elements to reverse order

B. Traverse the linked list and swap data values of nodes pairwise

C. Create a new linked list and push elements from old stack to new stack

D. Use recursion to pop all elements, then push them back in reverse order

Solution

1. Step 1: Understand constraints

   No extra data structures allowed, so using recursion (call stack) is allowed but no explicit new lists or queues.

2. Step 2: Recursion approach

   Recursively pop all elements, then insert them back at bottom to reverse order without extra storage.

3. Step 3: Other options use extra structures

   Swapping data or using new lists or queues violates constraints or is incorrect for stack reversal.

4. Final Answer:

   Use recursion to pop all elements, then push them back in reverse order -> Option D

5. Quick Check:

   Recursion reverses stack without extra structures [OK]

Hint: Use recursion to reverse stack without extra data structures [OK]

Common Mistakes:

• Using extra linked lists or queues
• Trying to swap node data incorrectly
• Ignoring no extra structure constraint