static public boolean isValid(String CCNumber) {
        int s1 = 0, s2 = 0;
        String reverse = new StringBuffer(CCNumber).reverse().toString();
        for(int i = 0 ; i < reverse.length(); i++){
            int digit = Character.digit(reverse.charAt(i), 10);
            if(i % 2 == 0) {
                s1 += digit;
            } else {
                s2 += 2 * digit;
                if(digit >= 5){
                    s2 -= 9;
                }
            }
        }
        return (s1 + s2) % 10 == 0;
    }