public class HelloWorld{

     public static void loopLetters(char begin, char end){
         System.out.print(begin);
         System.out.print(" ");

         if(begin != end)
            loopLetters(begin += (int) Math.signum(end - begin), end);
         else
            System.out.println();
     }

     public static void main(String []args){
        loopLetters('A', 'Z');
        loopLetters('a', 'z');
        loopLetters('Z', 'A');
        loopLetters('z', 'a');
        loopLetters('0', '9');
     }
}