public int evalRPN(String[] tokens) {
        Stack<Integer> ops = new Stack<>();
        int eval = 0, op1 = 0, op2 = 0;
        for(String token : tokens) {
            if(token.equals("+") || token.equals("-") || token.equals("*") || token.equals("/")) {
                op2 = ops.pop();
                op1 = ops.pop();
                switch(token) {
                    case "+":
                        eval = op1 + op2;
                        break;
                    case "-":
                        eval = op1 - op2;
                        break;
                    case "*":
                        eval = op1 * op2;
                        break;
                    case "/":
                        eval = op1 / op2;
                        break;
                }
                ops.push(eval);
            } else {
                ops.push(Integer.valueOf(token));
            }
        }
        
        return ops.pop();
    }