public static Tnode buildFromRPN (String pol) {
      List<Tnode> tList = correctExpression(pol);
      int counter = 0;

      while (tList.size() > 1) {
         if (tList.size() < 3) {
            throw new RuntimeException("Few invoices in statement");
         }

         if (isDigit(tList.get(counter)) || tList.get(counter).getFirstChild() != null) {
            if (isDigit(tList.get(counter + 1)) || tList.get(counter + 1).getFirstChild() != null) {
               if (!isDigit(tList.get(counter + 2)) && tList.get(counter + 2).getFirstChild() == null) {
                  Tnode top = tList.get(counter + 2);
                  Tnode left = tList.get(counter);
                  Tnode right = tList.get(counter + 1);
                  left.setNextSibling(right);
                  top.setFirstChild(left);
                  tList.remove(counter + 1);
                  tList.remove(counter);
                  counter = 0;
                  continue;
               }
            }
         }
         counter++;
      }
      return tList.get(0);
   }