public int evalRPN(String[] tokens) {
        Stack<Integer> s = new Stack() ;
        for(int i=0;i<tokens.length;i++){
            if(isOperators(tokens[i])){
                int va2 = s.pop();
                int va1 = s.pop();
                if("+".equals(tokens[i]))
                    s.push(va1+va2);
                else if("-".equals(tokens[i]))
                    s.push(va1-va2);
                else if("*".equals(tokens[i]))
                    s.push(va1*va2);
                else if("/".equals(tokens[i]))
                    s.push(va1/va2);
            }
            else
                s.push(Integer.parseInt(tokens[i]));
        }
        return s.pop();
    }