public Integer eval(String exp, int start) { Stack rator = new Stack<>(); Stack rands = new Stack<>(); for (int i = start; i < exp.length(); i++) {

        if (exp.charAt(i) == '+' || exp.charAt(i) == '-' || exp.charAt(i) == '*' || exp.charAt(i) == '/')  {
            rator.push(exp.charAt(i));
        } else if (exp.charAt(i) == ' ') {
            continue;
        } else if (Character.isDigit(exp.charAt(i))) {
            String num = "";
            while(Character.isDigit(exp.charAt(i))) {
                num += exp.charAt(i);
                i++;
            }
            rands.push(Integer.valueOf(num));
            i--;
        } else if (exp.charAt(i) == '(') {
            eval(exp, i+1);
        } else if (exp.charAt(i) == ')') {
            int expn = 0;
            while (!rator.isEmpty()) {
                Character op = rator.pop();
                Integer rand1 = rands.pop();
                Integer rand2 = rands.pop();
                rands.push(fun(op, rand1, rand2));
            }
        }
    }
    return Integer.valueOf(rands.pop());

}