What Is a Stack?

Overview

A stack is a simple data structure that stores items in a specific order. It follows the rule known as Last In, First Out, meaning the most recently added item is always the first one to be removed. This behavior is similar to stacking objects on top of each other.

LIFO Principle

LIFO stands for Last In, First Out. When you add an item to a stack, it is placed at the top. When you remove an item, the top element is always the one that leaves first. You cannot remove items from the middle or bottom of the stack.

Real-World Analogy

A stack works like a stack of books. You place books on top of each other, and when you want to remove a book, you take the one on the very top first. You never pull a book out from the bottom because it would cause everything above it to fall.

Core Operations

Stacks mainly support two essential operations. Push adds an item to the top of the stack. Pop removes the item currently sitting at the top. These two operations define how stacks behave and are the basis for many algorithms that rely on temporary storage or reversal of order.

When Stacks Are Useful

Stacks are used when actions must be reversed or undone in the exact opposite order they happened. Common examples include undo systems, backtracking algorithms, and storing the order of function calls during program execution.