두 갈래 나무가 층층이 다니고, 층층이 다니면서 두 갈래 나무가 완전한 두 갈래 나무인지 아닌지를 판단한다

 // ， null
    // ， 
    public static void levelOrderTraversal(TreeNode root) {
        //if (root == null) {
        //    return;
        //}

        Queue queue = new LinkedList<>();
        queue.add(root);

        while (!queue.isEmpty()) {
            TreeNode front = queue.poll();

            System.out.println(front.val);
            if (front.left != null) {
                queue.add(front.left);
            }
            if (front.right != null) {
                queue.add(front.right);
            }
        }
    }
    // 
    // 
    private  static class Element{
        TreeNode node;
        int level;
        // 
    }
    public List> levelOrder(TreeNode root){
        List> retList=new LinkedList<>();
        if (root == null) {
            return retList;
        }

        Queue queue = new LinkedList<>();
        Element e=new Element();
        e.node=root;
        e.level=0;
        queue.add(e);
        while(!queue.isEmpty()){
        Element front = queue.poll();

        if(front.level==retList.size()){
            retList.add(new ArrayList<>());
        }
        retList.get(front.level).add(front.node.val);

        if (front.node.left != null) {
            Element l = new Element();
            l.node = front.node.left;
            l.level = front.level + 1;
            queue.add(l);
            }
            if (front.node.right != null) {
                Element l = new Element();
                l.node = front.node.right;
                l.level = front.level + 1;
                queue.add(l);
            }
        }
        return retList;
    }
    // 
    public boolean isComplete(TreeNode root){
        Queue queue = new LinkedList<>();
        queue.add(root);
        // 
        while(true){
            TreeNode front=queue.poll();
            if(front==null){
                break;
            }
            queue.add(front.left);
            queue.add(front.right);
        }
        while (!queue.isEmpty()){
            TreeNode node =queue.poll();
            if(node!=null){
                return false;
            }
        }
        return true;
    }