public double polishEval(String polishExpression) {
        String[] tokens = polishExpression.split(" ");

    }

    boolean isOP(String token) {
        if (token.equals("+")) {
            return true;
        } else if (token.equals("-")) {
            return true;
        } else if (token.equals("*")) {
            return true;
        } else if (token.equals("/")) {
            return true;
        } else {
            return false;
        }
    }

    String strApply(String op, String v1, String v2) {
        if (op.equals("+")) {
            return Double.toString((Double.parseDouble(v1) + Double.parseDouble(v2)));
        } else if (op.equals("-")) {
            return Double.toString((Double.parseDouble(v1) - Double.parseDouble(v2)));
        } else if (op.equals("*")) {
            return Double.toString((Double.parseDouble(v1) * Double.parseDouble(v2)));
        } else if (op.equals("/")) {
            return Double.toString((Double.parseDouble(v1) / Double.parseDouble(v2)));
        } else {
            return "0";
        }
    }