StackToQueue.swift

import Foundation

class Object { 
    let identifier: String
    init(identifier: String) {
        self.identifier = identifier
    }
}

protocol MutableCollectionProtocol {
    var isEmpty: Bool { get }
    func push(_ some: Object)
    func pop() -> Object?
}

// First In Last Out
class Stack: MutableCollectionProtocol {
    private var array = [Object]() 
    
    var isEmpty: Bool { return array.isEmpty }
    
    func push(_ some: Object) { 
        array.append(some)
    }
    func pop() -> Object? { 
        return array.popLast()
    }
}

// First In First Out
class Queue: MutableCollectionProtocol {
    private var s1 = Stack()
    private var s2 = Stack()

    var isEmpty: Bool { fatalError() }

    func push(_ some: Object) { 
        fatalError()
    }
    func pop() -> Object? { 
        fatalError()
    }
}

var stack = Stack()
stack.push(Object(identifier: "1"))
stack.push(Object(identifier: "2"))
stack.push(Object(identifier: "3"))

// var queue = Queue()
// queue.push(Object(identifier: "1"))
// queue.push(Object(identifier: "2"))
// queue.push(Object(identifier: "3"))

let some: MutableCollectionProtocol = stack

let id1 = some.pop()?.identifier
print(id1)

let id2 = some.pop()?.identifier
print(id2)

// queue.push(Object(identifier: "4"))
// let id4 = some.pop()?.identifier
// print(id4)

let id3 = some.pop()?.identifier
print(id3)