Don't use “magic” values (in your case: -1) to indicate the absence of a value. Swift has the Optional type exactly for that purpose. The queue methods should be declared as
func push(_ x: Int)
func pop() -> Int?
func peek() -> Int?
func empty() -> Bool
and nil is returned if the queue is empty.
There is nothing special about the Int type here. It is simple to make the queue type generic
class MyQueue<Element> {
func push(_ x: Element)
func pop() -> Element?
func peek() -> Element?
func empty() -> Bool
}
and now you can use it with arbitrary types. For example
let queue = MyQueue<String>()
queue.push("foo")
There is no need for a dedicated front element, and the code becomes simpler without it.
Moving the elements from the first to the second stack in func pop() can be simplified to
while let elem = stack1.popLast() {
stack2.append(elem)
}
with a single test instead of two per iteration. Or even simpler:
stack2 = stack1.reversed()
stack1.removeAll()
(If your queues become really large then you might want to check which of the above methods is faster.)
If a function body consists of a single expression then the return keyword can be omitted.
Putting it all together, we get
class MyQueue<Element> {
private var stack1: [Element]
private var stack2: [Element]
init() {
stack1 = []
stack2 = []
}
func push(_ x: Element) {
stack1.append(x)
}
func pop() -> Element? {
if stack2.isEmpty {
stack2 = stack1.reversed()
stack1.removeAll()
}
return stack2.popLast()
}
func peek() -> Element? {
stack2.last ?? stack1.first
}
func empty() -> Bool {
stack1.isEmpty && stack2.isEmpty
}
}
which is shorter and simpler.
Finally, naming:
I had to inspect the code in order to understand that what you have implemented is a “first in, first out” queue. This would be more obvious to a user of your code (or to you in one year) with a more specific class name, such as FIFO or FIFOQueue.
Typical names for adding something to a FIFO and retrieving it are enqueue and dequeue.
Swift collection types typically use “isEmpty” as the name of a property which indicates whether the collection contains elements or not:
var isEmpty: Bool {
stack1.isEmpty && stack2.isEmpty
}
