stack in data structure: quick revision, 8 bites

Stack: LIFO at Its Core

Last in, first out — always.

A **stack** is a linear data structure where all insertions and deletions happen at one end — the **top**. It follows the **LIFO (Last In, First Out)** principle: the most recently added element is the first to be removed.

Takeaway. A stack only lets you access the top element — everything below is hidden until the top is removed.

What Is a Stack?

Think of a stack of plates.

Takeaway. A stack is a restricted data structure — you can only add or remove from the top.

Stack of Plates Analogy

Plates in a cafeteria — perfect LIFO.

Imagine a stack of plates in a cafeteria. You always **add** a clean plate on top, and you always **take** a plate from the top. You cannot remove a plate from the middle without first removing all plates above it.

Takeaway. The plate stack is the classic real-world model for LIFO behavior.

Undo Feature in Text Editors

Ctrl+Z uses a stack behind the scenes.

Takeaway. The undo feature is a classic stack application — the most recent action is undone first.

Confusing Stack with Queue

LIFO vs FIFO — don't mix them up.

Takeaway. Stack and queue have opposite removal orders — always check whether LIFO or FIFO fits your problem.

Stack Implementation in Python

Simple list-based stack in Python.

Python lists make stack operations trivial: `append()` for push, `pop()` for pop, and `list[-1]` for peek.

Takeaway. In Python, a list can act as a stack — just use append() and pop() with no arguments.

Browser Back Button

Every back click pops a URL.

Your browser's **back button** uses a stack to track visited pages. Each new page you visit is **pushed** onto the history stack. Clicking back **pops** the current page and reveals the previous one.

Takeaway. Browser history is a real-world stack — the most recent page is the first to go when you press back.

Stack in Expression Evaluation

Stacks solve parentheses and postfix.

In exams, stacks are frequently used to **check balanced parentheses** and **evaluate postfix expressions**. For parentheses: push '(' and pop when you see ')'. If the stack is empty at the end, it's balanced.

Takeaway. Stack-based parentheses checking is a common exam problem — practice it until it's automatic.