private String simpleMultiply(String num1, String num2) {
        String smaller;
        String larger;
        if (num1.length() == 1) {
            smaller = num1;
            larger = num2;
        } else {
            smaller = num2;
            larger = num1;
        }
        int r2 = smaller.charAt(0) - '0';
        if (r2 == 0) {
            return "0";
        }
        int carry = 0;
        StringBuffer stringBuffer = new StringBuffer();
        for (int i = larger.length() - 1; i >= 0; i--) {
            int r1 = larger.charAt(i) - '0';
            int r = r1 * r2 + carry;
            stringBuffer.append(r%10);
            carry = r / 10;
        }
        if (carry != 0) {
            stringBuffer.append(carry);
        }
        return stringBuffer.reverse().toString();
    }
https://leetcode.com/discuss/questions/oj/multiply-strings
src/com/interview/string/MultiplyStrings.java