bstLevelOrderQueuestack.swift

struct Queue {
    
    typealias NodeAtLevel = (node: TreeNode, level: Int)
    
    private var enqueueStack: [NodeAtLevel] = []
    private var dequeueStack: [NodeAtLevel] = []
    
    public var isEmpty: Bool {
        return enqueueStack.isEmpty && dequeueStack.isEmpty
    }
    
    public mutating func enqueue(_ node: TreeNode, at level: Int) {
        enqueueStack.append((node, level))
    }
    
    public mutating func dequeue() -> NodeAtLevel? {
        if dequeueStack.isEmpty {
            dequeueStack = enqueueStack.reversed()
            enqueueStack.removeAll()
        }
        return dequeueStack.popLast()
    }
}