Stack을 이용하여 Queue 만들기

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() -> String? {
        lock.lock()
        if outBox.isEmpty {
            while !inBox.isEmpty, let pop = inBox.pop() {
                outBox.push(item: pop)
            }
        }
        lock.unlock()
        
        return outBox.pop()
    }
    
    var isEmpty: Bool {
        return outBox.isEmpty
    }
}

// Testing

var queue = Queue()

(0...10)
    .map { String($0) }
    .forEach { item in
        queue.enQueue(item: item)
    }

print("item = \(queue.deQueue()!)")

queue.enQueue(item: "dh")

while let deQueue = queue.deQueue() {
    print("item = \(deQueue)")
}