code: private Object calculate(LinkedList<Object> el2){
          if(el2.peek() instanceof Operator){
              Operator obj = (Operator) el2.peek();
              return obj.calculate();
          }
          if(el2.peek() instanceof Elobj){
              return ((Elobj) el2.peek()).fetchVal();
          }
          return el2.peek();
      }
docs: 计算