public String printLevel(BSTNode node, int depth) {//PASS
		if (node == null) {
			return "";
		}
		else if(depth == 0) {
			String str = ""+node.data;
			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
	}