DSA · Theory

Double Ended Queue

Theory

26/810

Don’t start Double Ended Queue with jargon. Start with print jobs. That is the whole point.

Double Ended Queue on the board — enqueue / dequeue FIFO. Then say what print jobs looks like after.

Without Double Ended Queue, a lab printer gets messy and print jobs is hard to trust.

Use Double Ended Queue when print jobs must stay clear. If a simpler DSA step works, use that instead.

Don’t do this with Double Ended Queue — confusing front and rear. Interviewers spot it in ten seconds.

Place Double Ended Queue next to nearby DSA work — enqueue / dequeue FIFO is the link.

Close Double Ended Queue with — “If I skip it, print jobs goes wrong like this: confusing front and rear.”

Diagram

  front → [A][B][C] ← rear
  out ↑         ↑ in
       FIFO

Exam tip

Say Double Ended Queue in one breath, then enqueue / dequeue FIFO, then confusing front and rear.

from collections import deque

q = deque()
q.append(10)      # enqueue
q.append(20)
print(q.popleft())  # 10
print(list(q))

Double Ended Queue — queue is FIFO. deque.popleft() is O(1).