DSA · Theory

Reversing a Queue

Theory

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Reversing a Queue is a DSA topic. In plain words you use it for print jobs in a lab printer. Don’t start with a slogan — start with that picture.

Smallest example: enqueue / dequeue FIFO. Type it, run it, and say what you see. If you can do that from memory, you know Reversing a Queue.

From the example next to this theory — Reversing a Queue — queue is FIFO.

Trap — confusing front and rear. Fix that before you talk about advanced DSA.

Viva — what is Reversing a Queue? Then show enqueue / dequeue FIFO. Then name the trap.

Diagram

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

Exam tip

What is Reversing a Queue? Show this: enqueue / dequeue FIFO. Trap: confusing front and rear.

from collections import deque

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

Reversing a Queue — queue is FIFO. deque.popleft() is O(1).