## What Is the Stack in Data Structure?

In computer science, a stack is an abstract data type that follows the Last-In-First-Out (LIFO) principle. It can be visualized as a stack of plates, where you can only access the topmost plate.

The stack data structure is widely used in programming languages, compilers, operating systems, and various algorithms.

### Stack Operations

The stack supports two primary operations:

**Push**: This operation adds an element to the top of the stack.**Pop**: This operation removes and returns the element from the top of the stack.

Additionally, there are other useful operations associated with stacks:

__Peek__: This operation returns the topmost element without removing it.__IsEmpty__: This operation checks if the stack is empty or not.__Size__: This operation returns the number of elements in the stack.

### Implementation of a Stack

Stacks can be implemented using arrays or linked lists. Let’s take a look at both implementations:

#### Array-Based Stack Implementation:

```
<pre>
class Stack {
constructor() {
this.stack = [];
}
push(element) {
this.stack.push(element);
}
pop() {
if (!this.isEmpty()) {
return this.pop();
}
throw new Error('Stack is empty!');
}
peek() {
if (!this.stack[this.length - 1];
}
throw new Error('Stack is empty!');
}
isEmpty() {
return this.length === 0;
}
size() {
return this.length;
}
}
const stack = new Stack();
</pre>
```

#### Linked List-Based Stack Implementation:

`<pre>`

class Node {

constructor(data) {

this.data = data;

this.next = null;

}

}
class Stack {

constructor() {

this.top = null;

this.size = 0;

}

push(element) {

const newNode = new Node(element);

if (this.top === null) {

this.top = newNode;

} else {

newNode.next = this.top;

this.top = newNode;

}

this.size++;

}

pop() {

if (!this.isEmpty()) {

const removedNode = this.top;

if (this.size === 1) {

// Only one element in the stack

this.top = null;

} else {

// More than one element in the stack

this.top = removedNode.next;

}

removedNode.next = null; // Disconnect removed node from the stack

this.size--;

return removedNode.data;

}

throw new Error('Stack is empty!');

}

peek() {

if (!this.isEmpty()) {

return this.top.data;

}

throw new Error('Stack is empty!');

}

isEmpty() {

return this.size === 0;

}

size() {

return this.size;

}

}

### Applications of Stacks

Stacks are used in various real-world scenarios, including:

**Function Call Stack**: Stacks are used to keep track of function calls and their local variables. The order in which functions are called determines the order in which they will be executed.
**Undo/Redo Operations**: Many applications provide undo and redo functionality, where stacks are used to store the state of actions performed by the user.
**Expression Evaluation**: Stacks are utilized to evaluate arithmetic expressions, such as infix, postfix, and prefix notation.
**Backtracking Algorithms**: Stacks play a crucial role in backtracking algorithms like depth-first search (DFS).

Understanding stacks is fundamental for every programmer. By mastering this data structure, you can solve complex problems efficiently and write more optimized code.

Now that you have a solid understanding of stacks, it's time to apply this knowledge to your own projects.