public List<List<Integer>> printFromTopToBottom(TreeNode root) {
        List<List<Integer>> ans = new ArrayList<>();
        if(root == null) return ans;
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        boolean flag = true;
        while(!queue.isEmpty()){
            int size = queue.size();
            List<Integer> list = new ArrayList<>();
                while(size-- > 0) {
                    TreeNode node = queue.pollLast();
                    list.add(node.val);
                    if(node.left!=null) queue.addFirst(node.left);
                    if(node.right!=null) queue.addFirst(node.right);
                }
                flag = !flag;
            if(flag) Collections.reverse(list);
            ans.add(list);
        }
        return ans;
    }