code: @Override
      public int compare(String o1, String o2) {
  
          if (o1.equals(o2)) {
              return 0;
          }
  
          String prefix = Strings.commonPrefix(o1, o2);
          if (prefix.length() == 0) {
              // No common prefix
              return o1.compareTo(o2);
          }
  
          if (o1.length() == prefix.length()) {
              return -1;
          }
  
          return o1.compareTo(o2);
  
      }
docs: /foo < /foo/ < /foo/bar < /fooa
  
  @param o1
  @param o2
  @return