Stack Data Structure
Software Engineer path · lesson 4 of 99Understand the Last-In-First-Out (LIFO) principle. Visualize Push, Pop, and Peek operations.
Syntax: push n · pop · peek. Numbers from -99 to 99.
Empty stack (capacity 8).
- 1push(x): items.append(x) # top is the end
- 2pop(): if empty: underflow; return items.pop()
- 3peek(): return items[-1] # look, don’t remove
- Pushes
- 0 / 3
- Pops
- 0 / 2
Tap any element in the diagram to inspect its state at this step.
Keys: ← → step, Space play/pause. Predict mode pauses before key steps and asks what happens next.
Last in, first out
Run push 1, push 2, push 3, pop, push 4, pop, pop, pop. Order the values as they are popped.
Tap a piece, then tap where it goes — or drag it. Tap the tray to take one back.
- 1Drop a step here
- 2Drop a step here
- 3Drop a step here
- 4Drop a step here
Test your knowledge
Which element is removed first in a Stack?
Write the code
Valid Parentheses
function isValid(s) {
// Your code here
}The short version
Think of a stack of plates in a cafeteria. You can only add a new plate to the top, and you can only take the top plate off. The last one put on is the first one taken off (LIFO).
In more depth
Last In, First Out (LIFO)
Think of a stack of plates. You put a new plate on top, and you take the top plate off. You can't reach the bottom plate without removing all the ones above it.
This simple LIFO property makes Stacks essential for:
When to reach for this
- Processing nested structures (Parentheses, HTML tags)
- Reversing data (String reverse, number base conversion)
- Undo/Redo functionality
- Expression evaluation (Calculator)
Interview Questions
References & Further Reading
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