code: public Queue<Object> parseToRPN(String val) {
          rpn = new LinkedList<Object>();
          opts = new LinkedList<Operator>();
          
          Converter converter = new Converter(val);
          converter.initItems();
          while(!converter.isEnd()){
              Object item = converter.fetchItem();
              if(item instanceof Operator){
                  parseOperator((Operator) item);
                  continue;
              }
              rpn.add(item);
          }
          while(!opts.isEmpty()){
              rpn.add(opts.poll());
          }
          
          return rpn;
      }
docs: 转换成 逆波兰表示法（Reverse Polish notation，RPN，或逆波兰记法）
  @param val
  @throws IOException