Queue with two stacks

Playground.swift

var queue = Queue<String>()

queue.enqueue("Hello")
queue.enqueue("World")
queue.dequeue() // "Hello"
queue.dequeue() // "World"
queue.dequeue() // nil

Queue.swift

struct Queue<T> {
	var stack1 = Stack<T>()
	var stack2 = Stack<T>()

	mutating func enqueue(_ element: T) {
		stack1.push(element)
	}

	mutating func dequeue() -> T? {
		moveElements()
		return stack2.pop()
	}

	var isEmpty: Bool {
		return stack1.isEmpty && stack2.isEmpty
	}

	mutating func peek() -> T? {
		moveElements()
		return stack2.peek()
	}

	private mutating func moveElements() {
		guard stack2.isEmpty else { return }
		while let element = stack1.pop() {
			stack2.push(element)
		}
	}
}

Stack.swift

struct Stack<T> {
	var elements: [T] = []

	mutating func push(_ element: T) {
		elements.append(element)
	}

	mutating func pop() -> T? {
		return elements.popLast()
	}

	func peek() -> T? {
		return elements.last
	}

	func isEmpty: Bool {
		return elements.count == 0
	}
}