Stacks and Queues

Queue Enqueue/Dequeue

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

Algorithm

Basic Implementation

basic.swift 
func render(_ values: [Int]) -> String {
    values.map(String.init).joined(separator: " -> ")
}

var queue: [Int] = []
for value in [10, 20, 30] { queue.append(value) }
var removed: [Int] = []
while !queue.isEmpty { removed.append(queue.removeFirst()) }
print(render(removed))

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

Complexity

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

Implementation notes

  • var queue: [Int] = [] is a mutable Swift array used as the queue storage; the enqueue loop appends 10, 20, and 30 with queue.append(value).
  • var removed: [Int] = [] collects dequeued values so the replay can show the removal order separately from the remaining queue.
  • The loop guard while !queue.isEmpty prevents calling removeFirst() on an empty array, so there is no optional return or sentinel value in this source.
  • queue.removeFirst() returns the front Int and shifts the remaining array contents left; that front removal is linear for Swift Array.
  • The trace records queue moving from [] to [10, 20, 30], then the first dequeue producing removed = [10] and queue = [20, 30].
  • The remaining dequeues leave removed = [10, 20, 30] and queue = [].
  • render(_:) maps each Int to String and joins with " -> ", so print(render(removed)) outputs 10 -> 20 -> 30.
front The front is the oldest value still waiting in the queue.
FIFO A queue removes values in first-in, first-out order.