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 Swift 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.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))

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.