code: public static String copy(String str, int count) {
          if (str == null) {
              throw new NullPointerException("原始字符串不能为null");
          }
  
          if (count <= 0) {
              throw new IllegalArgumentException("次数必须大于0");
          }
  
          if (count == 1) {
              return str;
          }
  
          if (count == 2) {
              return str + str;
          }
  
          StringBuilder sb = new StringBuilder();
          for (int i = 0; i < count; i++) {
              sb.append(str);
          }
          return sb.toString();
      }
docs: 将目标字符串重复count次返回
  @param str 目标字符串
  @param count 次数
  @return 目标字符串重复count次结果，例如目标字符串是test，count是2，则返回testtest，如果count是3则返回testtesttest