public int evalRPN(String[] tokens) {
        Stack<String> stack = new Stack<String>();
        for(String token: tokens){
            if(validOperators.contains(token)){
                int operand2 = Integer.parseInt(stack.pop());
                int operand1 = Integer.parseInt(stack.pop());
                if("+".equals(token))
                    stack.push(String.valueOf(operand1 + operand2));
                else if("-".equals(token))
                    stack.push(String.valueOf(operand1 - operand2));
                else if("*".equals(token))
                    stack.push(String.valueOf(operand1 * operand2));
                else if("/".equals(token))
                    stack.push(String.valueOf(operand1 / operand2));
            }else{
                stack.push(token);
            }
        }
        return Integer.parseInt(stack.pop());
    }