public static String printLevel(TreeNode node, int depth) {//PASS
        if (node == null) {
            return "";
        }
        else if(depth == 0) {
            String str = ""+node.val;
            return str;
        }
        else if (depth > 0) {
            String leftStr = printLevel(node.left, depth-1);//the depth decres
            String rightStr = printLevel(node.right, depth-1);
            return leftStr+"_"+rightStr;
        }
        else return "";//default case
    }