swift – 无法将Node类型的值分配给Node？

class Node<T> {
    var key: T?
    weak var next: Node<T>?
    weak var prevIoUs: Node<T>?

    init(key: T,prevIoUs: Node? = nil) {
        self.key = key
        self.prevIoUs = prevIoUs
    }
}


class Dequeue<T> {
    private var count: Int = 0
    private weak var top: Node<T>?
    private weak var bottom: Node<T>?

    func enqueue<T>(val: T) {
       // if dequeue is empty
       let node = Node<T>(key: val)

        if top == nil {
            self.top = node
            self.bottom = node
        } else {
            self.bottom?.next = node
            self.bottom = node
        }
     }

}