A stack is a vertical container. You only ever interact with the top. It's the simplest restriction on an array, yet it powers the most complex software.

The Cafeteria Tray Analogy

Think of a stack of trays in a cafeteria. When a new tray is washed, it goes on **top**. When a hungry student needs a tray, they take the one from the **top**.

Key Rule: Single Exit/Entry

You cannot pull a tray from the middle without causing a mess. Every interaction happens at one specific point: **The Top**.

TRAY 3 (NEWEST)

TRAY 2

TRAY 1 (OLDEST)

Base of Stack

Efficiency

All primary operations occur in **O(1) constant time**. No searching, shifting, or loops required.

Dynamic

Stacks usually grow and shrink as needed, though they can be implemented with fixed-size arrays.

Abstract

A stack is an **Abstract Data Type (ADT)**. It defines behavior, not how the internal bits are stored.

Module Overview Level 2: Stack Operations