StringBuilder add(StringBuilder a, StringBuilder b) {
        int ii = a.length() - 1;
        int jj = b.length() - 1;
        StringBuilder nr = new StringBuilder();
        int q = 0;
        while (ii >= 0 || jj >= 0) {
            int sum = q;
            if (ii >= 0 && jj >= 0) {
                sum += (a.charAt(ii) - '0') + (b.charAt(jj) - '0');
            } else if (ii >= 0) {
                sum += (a.charAt(ii) - '0');
            } else {
                sum += (b.charAt(jj) - '0');
            }
            nr.append(sum % 10);
            q = sum / 10;

            ii--;
            jj--;
        }
        if (q != 0)
            nr.append(q);
        return nr.reverse();
    }
Why Did you create this class? what does it do?
src/problems/medium/MultiplyStrings.java