Stacks and Queues

Queue Enqueue/Dequeue

Enqueue values at the back and dequeue them from the front in first-in, first-out order.

Algorithm

The replay uses the same three values in every language, so this Python DSA implementation can be compared directly with the rest of the DSA track.

front The front is the oldest value still waiting in the queue.
FIFO A queue removes values in first-in, first-out order.

Visual walkthrough

The queue keeps the oldest value at the front and adds new values at the back.

Step 1 - Enqueue 10, 20, 30

New values join at the back. The oldest value, 10, stays at the front.

Queue after three enqueues: front 10, then 20, then back 30.nextnext10front2030back

Step 2 - Dequeue removes 10

Removing from the front returns 10 and makes 20 the new front.

After one dequeue: removed is 10; front moves to 20.next10removed20front30back

Basic Implementation

basic.py 
from collections import deque

queue = deque()
for value in [10, 20, 30]:
    queue.append(value)
removed = []
while queue:
    removed.append(queue.popleft())
print(" -> ".join(str(x) for x in removed))

Complexity

  • Time: O(1) per operation with a real queue
  • Space: O(n)

Implementation notes

  • Python uses collections.deque for the queue, not a list with pop(0). queue.append(value) mutates the right end, and queue.popleft() removes and returns the oldest value from the left in O(1) time.
  • The deque and the removed list are separate containers. Each removed.append(queue.popleft()) transfers the returned int reference into removed while shrinking the deque.
  • The visible containers are the deque and the removed list; normal Python GC owns them once no references remain. The trace shows the queue move from [] to [10, 20, 30], then drain into removed.