// is used to test your code on a small example
    public static void main2(String args[]) {
        String test[] = {"0010110", "1001001", "0110010", "1001000", "0100001", "0001101"};
        int r = test.length, c = test[0].length();
        ExactCover e = new ExactCover(c);
        for (int i=0; i<r; i++)
            for (int j=0; j<c; j++)
                if (test[i].charAt(j)=='1')
                    e.add(i,j);
        if (e.solve()) 
            for (int i: e.solution)
                System.out.println(i+": "+test[i]);
        else System.out.println("no solution");
    }