Stacks – LIFO Structure

Before You Apply

First observe the behavior in Explore mode, then switch to Apply to test each operation with intent.

Core Idea

A stack is a linear data structure that follows the Last-In-First-Out (LIFO) principle. Elements are added (pushed) and removed (popped) from the same end called the top.

Observe This First

LIFO (Last-In-First-Out): Last element added is first to be removed

Then Apply

Browser History: Back button uses stack

Stacks – LIFO Structure Lab

Only the top can be removed next. Click blocks to inspect stack positions.

TOP

Ready|Waiting for a stack operation

Time: Depends on operation|Space: O(1) auxiliary

Current

Comparing

Correct / Inserted

Removed

Pseudocode

1peek top element

2prepare new node on top

3push value onto top

4mark top for pop

5remove top and reveal previous element

6visit stack from top to bottom

Stacks are LIFO: the top is always the next value to leave.

Explore mode: inspect the top element to understand LIFO behavior.

Understanding the Observation

What did you observe?

Push: Add element to top → O(1)

Why did this happen?

LIFO (Last-In-First-Out): Last element added is first to be removed

Apply with purpose

In Apply mode, test one operation and explain its complexity before running the next.

Predict next

Before clicking run, predict the next index/pointer movement and then verify it.