Starting from an empty queue, what do enqueue(5), enqueue(3), dequeue(), enqueue(7), dequeue(), first(), dequeue(), dequeue() return?

Question

Accepted Answer

dequeue() → 5, dequeue() → 3, first() → 7, dequeue() → 7, and the last dequeue() → null because the queue is empty.

* dequeue always takes the oldest element still waiting. *

Operation
Returns
Queue (front … rear)

enqueue(5)
—
(5)

enqueue(3)
—
(5, 3)

dequeue()
5
(3)

enqueue(7)
—
(3, 7)

dequeue()
3
(7)

first()
7
(7)

dequeue()
7
()

dequeue()
null
()

Compare with the same kind of sequence on a stack: there, the first removal would have returned 3, the most recent element. Here it returns 5, the oldest. first() peeks without removing, so the following dequeue() returns the same 7. On an empty queue, this ADT returns null rather than throwing.

Operation	Returns	Queue (front … rear)
`enqueue(5)`	—	(5)
`enqueue(3)`	—	(5, 3)
`dequeue()`	5	(3)
`enqueue(7)`	—	(3, 7)
`dequeue()`	3	(7)
`first()`	7	(7)
`dequeue()`	7	()
`dequeue()`	`null`	()