DSA Stacks

DSA Stacks – Complete Beginner Guide

(Concept • Operations • Implementation • Examples • Complexity)

A Stack is one of the most important linear data structures in DSA.
It follows a simple rule called LIFO.

LIFO = Last In, First Out

1️⃣ What is a Stack?

A Stack is a data structure where:

Insertion happens at one end only (TOP)
Deletion happens from the same end

Real-Life Examples

Stack of plates 🍽️
Undo / Redo in editors
Browser back button
Function calls (Call Stack)

2️⃣ Stack Terminology

Term	Meaning
Top	Points to the last element
Push	Insert element
Pop	Remove element
Peek / Top	View top element
Overflow	Stack full
Underflow	Stack empty

3️⃣ Basic Stack Operations

1. Push (Insert)

Add element at the top.

2. Pop (Delete)

Remove element from the top.

3. Peek

View the top element without removing it.

4. isEmpty

Check if stack is empty.

4️⃣ Stack Representation (Array)

5️⃣ Stack Implementation Using Array (C++)

#include <iostream>
using namespace std;

#define MAX 5
int stack[MAX];
int top = -1;

// Push
void push(int value) {
if(top == MAX - 1) {
cout << "Stack Overflow\n";
return;
}
stack[++top] = value;
}

// Pop
void pop() {
if(top == -1) {
cout << "Stack Underflow\n";
return;
}
cout << "Popped: " << stack[top--] << endl;
}

// Peek
void peek() {
if(top == -1) {
cout << "Stack is Empty\n";
return;
}
cout << "Top element: " << stack[top] << endl;
}

int main() {
push(10);
push(20);
push(30);

peek();
pop();
peek();

return 0;
}

#include <iostream>

using namespace std;

#define MAX 5

int stack[MAX];

int top = -1;

// Push

void push(int value) {

if(top == MAX - 1) {

cout << "Stack Overflow\n";

return;

}

stack[++top] = value;

}

// Pop

void pop() {

if(top == -1) {

cout << "Stack Underflow\n";

return;

}

cout << "Popped: " << stack[top--] << endl;

}

// Peek

void peek() {

if(top == -1) {

cout << "Stack is Empty\n";

return;

}

cout << "Top element: " << stack[top] << endl;

}

int main() {

push(10);

push(20);

push(30);

peek();

pop();

peek();

return 0;

}

Output

6️⃣ Stack Implementation Using Linked List

#include <iostream>
using namespace std;

struct Node {
int data;
Node* next;
};

Node* top = NULL;

// Push
void push(int value) {
Node* newNode = new Node();
newNode->data = value;
newNode->next = top;
top = newNode;
}

// Pop
void pop() {
if(top == NULL) {
cout << "Stack Underflow\n";
return;
}
Node* temp = top;
cout << "Popped: " << temp->data << endl;
top = top->next;
delete temp;
}

// Peek
void peek() {
if(top == NULL) {
cout << "Stack is Empty\n";
return;
}
cout << "Top element: " << top->data << endl;
}

int main() {
push(10);
push(20);
push(30);

peek();
pop();
peek();

return 0;
}

#include <iostream>

using namespace std;

struct Node {

int data;

Node* next;

};

Node* top = NULL;

// Push

void push(int value) {

Node* newNode = new Node();

newNode->data = value;

newNode->next = top;

top = newNode;

}

// Pop

void pop() {

if(top == NULL) {

cout << "Stack Underflow\n";

return;

}

Node* temp = top;

cout << "Popped: " << temp->data << endl;

top = top->next;

delete temp;

}

// Peek

void peek() {

if(top == NULL) {

cout << "Stack is Empty\n";

return;

}

cout << "Top element: " << top->data << endl;

}

int main() {

push(10);

push(20);

push(30);

peek();

pop();

peek();

return 0;

}

7️⃣ Time Complexity of Stack Operations

Operation	Time
Push	O(1)
Pop	O(1)
Peek	O(1)
Search	O(n)

8️⃣ Stack Overflow & Underflow

Stack Overflow

Occurs when pushing into a full stack.

Stack Underflow

Occurs when popping from an empty stack.

9️⃣ Applications of Stack

✔ Function calls (Call Stack)
✔ Expression evaluation
✔ Parenthesis checking
✔ Undo / Redo operations
✔ Depth First Search (DFS)
✔ Reversing data

🔟 Stack vs Queue

Feature	Stack	Queue
Principle	LIFO	FIFO
Insert	Push	Enqueue
Delete	Pop	Dequeue
Use case	Undo, DFS	Scheduling, BFS

📌 Interview Questions (Stack)

Q1. What is stack?
👉 Linear data structure using LIFO

Q2. Is stack static or dynamic?
👉 Can be both

Q3. Stack overflow condition?
👉 When stack is full

Q4. Stack applications?
👉 Recursion, expression evaluation

✅ Final Summary

✔ Stack follows LIFO principle
✔ Insert & delete from same end
✔ Easy to implement
✔ O(1) operations
✔ Very important for DSA interviews

DSA Stacks

DSA Stacks – Complete Beginner Guide

(Concept • Operations • Implementation • Examples • Complexity)

1️⃣ What is a Stack?

Real-Life Examples

2️⃣ Stack Terminology

3️⃣ Basic Stack Operations

1. Push (Insert)

2. Pop (Delete)

3. Peek

4. isEmpty

4️⃣ Stack Representation (Array)

5️⃣ Stack Implementation Using Array (C++)

Output

6️⃣ Stack Implementation Using Linked List

7️⃣ Time Complexity of Stack Operations

8️⃣ Stack Overflow & Underflow

Stack Overflow

Stack Underflow

9️⃣ Applications of Stack

🔟 Stack vs Queue

📌 Interview Questions (Stack)

✅ Final Summary

You may also like...

DSA Tutorial

Arrays

Linked Lists

Stacks & Queues

Hash Tables

Trees

Graphs

Shortest Path

DSA Stacks

DSA Stacks – Complete Beginner Guide

(Concept • Operations • Implementation • Examples • Complexity)

1️⃣ What is a Stack?

Real-Life Examples

2️⃣ Stack Terminology

3️⃣ Basic Stack Operations

1. Push (Insert)

2. Pop (Delete)

3. Peek

4. isEmpty

4️⃣ Stack Representation (Array)

5️⃣ Stack Implementation Using Array (C++)

Output

6️⃣ Stack Implementation Using Linked List

7️⃣ Time Complexity of Stack Operations

8️⃣ Stack Overflow & Underflow

Stack Overflow

Stack Underflow

9️⃣ Applications of Stack

🔟 Stack vs Queue

📌 Interview Questions (Stack)

✅ Final Summary

You may also like...

DSA Queues

DSA Tutorial

Arrays

Linked Lists

Stacks & Queues

Hash Tables

Trees

Graphs

Shortest Path