a recursive program to write plot the subdivisions of a ruler using StdDraw library, <introduction to programming in java an interdisciplinary approach

Ruler.java

public class Ruler
{
    public static String ruler(int N)
    {
	if (N <= 0) return "";
	if (N == 1) return "1";
	return ruler(N-1) + N + ruler(N-1);
    }
   
    public static void drawRuler(String a)
    {
	String[] b = a.split("");
	StdDraw.setXscale(0, b.length);
	for(int i = 1, j = 0; i < b.length; i++, j++)
	    {
		int d = Integer.parseInt(b[i]);
		StdDraw.line(j, 0, j, d);
	    }

    }
    public static void main(String[] args)
    {
	int N = Integer.parseInt(args[0]);
	StdDraw.setYscale(0, N);
	String a = ruler(N);
	drawRuler(a);
    }
}