Pop Operation on Stack in DSA C#

Mental Model

A stack is like a pile where you only take the top item off. Pop means removing that top item.

Analogy: Imagine a stack of plates. You always take the top plate first when you need one. Popping is like removing that top plate.

Top -> [3] -> [2] -> [1] -> null

Dry Run Walkthrough

Input: stack: 1 -> 2 -> 3 (top is 3), perform pop operation

Goal: Remove the top element (3) from the stack and show the new top

Step 1: Check if stack is empty

Top -> [3] -> [2] -> [1] -> null

Why: We must ensure there is something to pop

Step 2: Remove the top element (3) and update top to next node

Top -> [2] -> [1] -> null

Why: Popping removes the top and moves top pointer down

Step 3: Return the popped value (3)

Top -> [2] -> [1] -> null

Why: We want to know which element was removed

Result:

Top -> [2] -> [1] -> null
Popped value: 3

Annotated Code

DSA C#

using System;

public class Node
{
    public int Data;
    public Node Next;

    public Node(int data)
    {
        Data = data;
        Next = null;
    }
}

public class Stack
{
    private Node top;

    public Stack()
    {
        top = null;
    }

    public void Push(int data)
    {
        Node newNode = new Node(data);
        newNode.Next = top; // link new node to current top
        top = newNode; // update top to new node
    }

    public int Pop()
    {
        if (top == null) // check if stack is empty
        {
            throw new InvalidOperationException("Stack is empty");
        }
        int poppedValue = top.Data; // store top data
        top = top.Next; // move top pointer down
        return poppedValue; // return popped value
    }

    public void PrintStack()
    {
        Node current = top;
        while (current != null)
        {
            Console.Write(current.Data + " -> ");
            current = current.Next;
        }
        Console.WriteLine("null");
    }
}

public class Program
{
    public static void Main()
    {
        Stack stack = new Stack();
        stack.Push(1);
        stack.Push(2);
        stack.Push(3);

        stack.PrintStack();
        int popped = stack.Pop();
        Console.WriteLine("Popped value: " + popped);
        stack.PrintStack();
    }
}

if (top == null) // check if stack is empty

ensure stack is not empty before popping

int poppedValue = top.Data; // store top data

save the value to return after removal

top = top.Next; // move top pointer down

remove top by moving pointer to next node

return poppedValue; // return popped value

output the removed element

Output

3 -> 2 -> 1 -> null

Popped value: 3

2 -> 1 -> null

Complexity Analysis

Time: O(1) because popping only changes the top pointer once

Space: O(1) because no extra space is used during pop

vs Alternative: Compared to arrays where removing last element is O(1), linked list stack pop is also O(1) but avoids resizing overhead

Edge Cases

empty stack

throws InvalidOperationException to prevent popping from empty stack

DSA C#

if (top == null) // check if stack is empty

stack with one element

pop removes the only element and top becomes null

DSA C#

top = top.Next; // move top pointer down

When to Use This Pattern

When you see a problem needing last-in-first-out order, reach for stack pop operation because it removes the most recent item quickly.

Common Mistakes

Mistake: Not checking if the stack is empty before popping
Fix: Add a check for empty stack and handle it (throw exception or return special value)

Mistake: Not updating the top pointer after popping
Fix: Set top to top.Next to remove the top element properly

Summary

Removes the top element from the stack and returns it.

Use when you need to process items in reverse order of insertion.

The key is to update the top pointer to the next node to remove the current top.

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