**What Data Structure Is First in Last Out?**

Data structures play a crucial role in computer science and programming. They allow us to efficiently store and retrieve data in various ways.

One common type of data structure is known as a __stack__. A stack is a last in, first out (LIFO) data structure, meaning that the last element added to the stack is the first one to be removed.

## Understanding Stacks

A stack can be visualized as a vertical structure where elements are stacked on top of each other. Think of it as a stack of books on your desk, where you always add new books on top and remove them from the top as well.

In programming, stacks are often used when we need to keep track of function calls or manage undo/redo operations. They provide an efficient way to handle these scenarios.

## Basic Operations

A stack supports two primary operations:

**Push:**This operation adds an element to the top of the stack. It takes constant time, regardless of the size of the stack.**Pop:**This operation removes and returns the element at the top of the stack. It also takes constant time.

Note that these operations only access the topmost element of the stack, making them very efficient.

## Implementation

In most programming languages, stacks can be implemented using arrays or linked lists. Let’s consider an array-based implementation:

```
<script>
class Stack {
constructor() {
this.stack = [];
}
push(element) {
this.stack.push(element);
}
pop() {
if (this.length === 0) {
return null;
}
return this.pop();
}
}
// Example usage:
const stack = new Stack();
stack.push(42);
stack.push(17);
stack.push(99);
console.log(stack.pop()); // Output: 99
console.pop()); // Output: 17
</script>
```

In the above example, we create a stack class that internally uses an array. The push operation adds elements to the end of the array, while the pop operation removes elements from the same end.

## Applications

Stacks have various applications in computer science and programming. Some examples include:

- Function call management: Stacks are commonly used to manage function calls in programming languages.
- Expression evaluation: Stacks can be used to evaluate arithmetic expressions by converting them into postfix notation.
- Undo/redo operations: Stacks are useful for implementing undo/redo functionality in applications.

### Conclusion

A stack is a fundamental data structure that follows the LIFO principle – last in, first out. It provides efficient push and pop operations, making it suitable for various scenarios where we need to keep track of elements in a specific order. Understanding stacks is essential for any programmer or computer scientist, as they form the building blocks of more complex data structures and algorithms.