public static ArrayList<String> getpowerset(int a[],int n,ArrayList<String> ps)
    {
        if(n<0)
        {
            return null;
        }
        if(n==0)
        {
            if(ps==null)
                ps=new ArrayList<String>();
            ps.add(" ");
            return ps;
        }
        ps=getpowerset(a, n-1, ps);
        ArrayList<String> tmp=new ArrayList<String>();
        for(String s:ps)
        {
            if(s.equals(" "))
                tmp.add(""+a[n-1]);
            else
                tmp.add(s+a[n-1]);
        }
        ps.addAll(tmp);
        return ps;
    }
