Queue Basics 10 XP Easy

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A Queue is a First-In-First-Out (FIFO) data structure — just like waiting in line at a store! 🏪

💡 Think of it like this: Imagine a line at a coffee shop. The first person in line gets served first. New people join at the back. No cutting! That's a queue.

from collections import deque

q = deque()
q.append('Alice')    # Joins line (back)
q.append('Bob')      # Joins line (back)
q.append('Charlie')  # Joins line (back)

print(q.popleft())   # Served first → 'Alice'
print(q.popleft())   # Served next → 'Bob'
print(q[0])          # Who's next? → 'Charlie'

Queue operations (all O(1) with deque):

q.append(x) — Enqueue: add to back of line
q.popleft() — Dequeue: remove from front (served!)
q[0] — Peek: see who's at the front
⚠️ NEVER use list.pop(0) — it's O(n)! Always use deque

Where queues are used:

🌊 BFS (Breadth-First Search) — Level-by-level tree/graph traversal
🖨️ Print queue — Documents print in order they were sent
📱 Task scheduling — OS runs processes in queue order
💬 Message queues — Chat apps deliver messages in order

📋 Instructions

Practice using a queue with deque. Enqueue values 'A', 'B', 'C', 'D'. Dequeue twice and print each. Then print the front element.

Use deque(), append() to enqueue, popleft() to dequeue, and q[0] to peek at the front.

🧪 Test Cases

Input

q.popleft()

Expected

A

Test case 1

Input

q.popleft()

Expected

B

Test case 2

Input

q[0]

Expected

C

Test case 3

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main.py

Hi! I'm Rex 👋

Output

Ready. Press ▶ Run or Ctrl+Enter.

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