Data Structures: Stack

Explanation Objective first: Understand the foundational LIFO principle and physical structure of a stack.

Instructions: Read each card to discover how stacks store and retrieve data.

Step 1 of 3

The Big Picture

A stack is a linear data structure that models a physical pile of items, such as a stack of plates or cafeteria trays. Elements are placed directly on the top, and items can only be retrieved from that very same top position.
In a stack of trays, the tray washed and added last sits on top and is taken first by the next person.
Step 2 of 3

The Key Connection: LIFO Order

The organizing rule of every stack is Last-In, First-Out (LIFO). The element inserted most recently is always the very first element to be removed. Consequently, the first item placed onto an empty stack must wait until all subsequent items above it are cleared.
If you push items A, B, and then C onto an empty stack, item C is at the top and must be removed before B or A can be accessed.
Step 3 of 3

From Idea to Practice: Restricted Access

Unlike arrays that allow reading any element by index, a stack intentionally restricts all access to a single point called the top. This focused constraint makes stack operations predictable, secure, and fast for tracking sequential history.
You cannot directly extract the bottom item of a stack without first removing all items resting above it.