请问为什么我按照视频来写的代码，最后运行结果还是有很多-1呢？我的代码如下：
let queue = DispatchQueue(label: "myQueue", qos: DispatchQoS.default, attributes: DispatchQueue.Attributes.concurrent, autoreleaseFrequency: DispatchQueue.AutoreleaseFrequency.inherit, target: nil)
let queue2 = DispatchQueue(label: "myQueue2", qos: DispatchQoS.default, attributes: DispatchQueue.Attributes.concurrent, autoreleaseFrequency: DispatchQueue.AutoreleaseFrequency.inherit, target: nil)
let arrayQueue = DispatchQueue(label: "myQueue3", qos: DispatchQoS.default, attributes: DispatchQueue.Attributes.concurrent, autoreleaseFrequency: DispatchQueue.AutoreleaseFrequency.inherit, target: nil)

var array = Array(1...1000)

func getLastItem() -> Int {
    return arrayQueue.sync { () -> Int in
        if array.count > 0 {
            return array[array.count - 1]
        }
        return -1
    }
}

func removeLastItem() {
    let workItem = DispatchWorkItem(qos: DispatchQoS.default, flags: DispatchWorkItemFlags.barrier) {
        array.removeLast()
    }
    arrayQueue.async(execute: workItem)
}

queue.async {
    for _ in 0..<1000 {
        removeLastItem()
    }
}

queue2.async {
    for _ in 0..<1000 {
        print(getLastItem())
    }
}