Stack Basics 10 XP Easy

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A Stack is a Last-In-First-Out (LIFO) data structure — like a stack of plates! 🍽️

💡 Think of it like this: Imagine stacking plates. You always put new plates on TOP and take from the TOP. The last plate you put on is the first one you take off. That's LIFO!

# Python lists work perfectly as stacks!
stack = []
stack.append(10)   # Push: add to top
stack.append(20)
stack.append(30)
print(stack[-1])    # Peek: look at top → 30
stack.pop()         # Pop: remove from top → 30
# stack is now [10, 20]

Stack operations (all O(1)):

stack.append(x) — Push: add element to the top
stack.pop() — Pop: remove and return top element
stack[-1] — Peek: look at top without removing
len(stack) == 0 — isEmpty: check if stack is empty

Where stacks appear everywhere:

↩️ Undo/Redo: Ctrl+Z uses a stack (last action undone first)
🌐 Browser history: Back button pops from the history stack
🔤 Matching brackets: The classic stack interview problem
📞 Function calls: Your computer uses a "call stack" to track function calls
🧮 Expression evaluation: Calculators use stacks for order of operations

📋 Instructions

Practice using a stack. Push values 5, 10, 15, 20 onto a stack. Pop twice and print each popped value. Then print the top of the remaining stack using peek.

Use append() to push, pop() to remove from top, and stack[-1] to peek at the top without removing.

🧪 Test Cases

Input

stack.pop()

Expected

20

Test case 1

Input

stack.pop()

Expected

15

Test case 2

Input

stack[-1]

Expected

10

Test case 3

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main.py

Hi! I'm Rex 👋

Output

Ready. Press ▶ Run or Ctrl+Enter.

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