25、按层打印二叉树《剑指offer》

从上往下打印出二叉树的每个节点，同层节点从左至右打印。

    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
            ArrayList<Integer> list=new ArrayList<>();
            if(root==null){
                        return list;
            }
        Queue<TreeNode> queue=new LinkedList<>();
        queue.offer(root);
        TreeNode current;
        while(!queue.isEmpty()){
            current=queue.poll();
            list.add(current.val);
            if(current.left!=null){
                        queue.offer(current.left);
            }
            if(current.right!=null){
                        queue.offer(current.right);
            }
    }
        return  list;
}