code: private LinkedList<Object> OperatorTree(Queue<Object> rpn){
          LinkedList<Object> operand = new LinkedList<Object>();
          while(!rpn.isEmpty()){
              if(rpn.peek() instanceof Operator){
                  Operator opt = (Operator) rpn.poll();
                  opt.wrap(operand);
                  operand.addFirst(opt);
                  continue;
              }
              if(rpn.peek() instanceof Elobj){
                  ((Elobj) rpn.peek()).setEc(ec);
              }
              operand.addFirst(rpn.poll());
          }
          return operand;
      }
docs: 转换成操作树