@Override

import Foundation class Stack { private var list = [String]() func push(item: String) { list.append(item) } func pop() -> String? { return list.popLast() } var isEmpty: Bool { return list.isEmpty } } class Queue { private var inBox = Stack() private var outBox = Stack() private let lock = NSLock() func enQueue(item: String) { lock.lock() inBox.push(item: item) lock.unlock() } func deQueue() -> S..