Queue Basics

The Idea

A queue is open at both ends. Items join at the back and leave from the front, so the oldest one is always served first. Reach for collections.deque: removing the front is O(1) there, while list.pop(0) costs O(n) because everything behind it shifts down a slot.

Real-World Example

A shared office printer. Three people hit print in the same minute and the machine works through the documents in the order they arrived. A fourth job sent now waits behind all of them, however urgent its owner feels.

The Code

from collections import deque

q = deque()
q.append("job1")         # enqueue at the back -> O(1)
q.append("job2")
q.append("job3")

print(q[0])              # "job1" peek at the front
print(q.popleft())       # "job1" the oldest leaves first
print(q.popleft())       # "job2"
print(list(q))           # ["job3"]
# a plain list would need pop(0) here, which is O(n)

from collections import deque
q = deque(["a", "b", "c"])
# take out the item that arrived first
first = q.___()