public static String reverseWords(String s) {
        /*
          Assume that the parameter String can only contain spaces and alphanumeric characters.
          Please implement this method to
          reverse each word in the original String while maintaining the word order.
          For example:
          parameter: "Hello world", result: "olleH dlrow"
         */
        StringBuffer result = new StringBuffer();
        StringTokenizer st = new StringTokenizer(s);
        while (st.hasMoreTokens()) {
            String reverseToken = reverse(st.nextToken());
            result.append(reverseToken).append(" ");
        }
        return result.toString();
    }