code: private static String join(String str, int times, String sep) {
      	StringBuilder sb = new StringBuilder();
      	for(int i = 0; i < times; i++) {
      		sb.append(str);
      		if(i < times - 1) {
      			sb.append(sep);
      		}
      	}
      	return sb.toString();
      }
docs: 例如：str=?,times=3,sep=,  返回 ?,?,?