Codewar-008: Playing with passphrases 玩玩加密口令

https://www.codewars.com/kata/559536379512a64472000053/train/java

package codewar;
public class PlayPass{
  private static String alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
  public static String playPass(String s, int n){
    StringBuilder sb = new StringBuilder();
    for(int i = 0; i < sb.length(); i++){
      char x = sb.charAt(i);

      if(Character.isLetter(x)){
        int idx = alphabet.indexOf(x);
        int origin = (idx + n) % 26;
        sb.setCharAt(i, alphabet.charAt(origin));
      }
      if(Character.isDigit(x)){
        int d = 9 - Integer.valueOf(String.valueOf(x));
        sb.setCharAt(i, String.valueOf(d).charAt(0));
      }
    }
    for(int i = 1; i < sb.length(); i = i + 2){
      char x = sb.charAt(i);
      if(x >= 65 && x <= 90){
        sb.setCharAt(i, (char)(x + 32));
      }
    }
    sb.reverse();
    return sb.toString();
  }
}