BinaryTree+LevelOrder.swift

extension BinaryTree {

    func levelOrder() {
        guard let node = self.rootNode else { return }
        self.queue.enqueue(item: node)
        while !self.queue.isEmpty {
            guard let dequeueNode = self.queue.dequeue() else { break }

            if let leftNode = dequeueNode.leftNode {
                self.queue.enqueue(item: leftNode)
            }

            if let rightNode = dequeueNode.rightNode {
                self.queue.enqueue(item: rightNode)
            }
        }
    }
}