public class Solution {
enum Operation{
        ADD('+'){int operate(int a,int b){return a+b;}},
        MINUS('-'){int operate(int a,int b){return a-b;}},
        MULTIPLY('*'){int operate(int a,int b){return a*b;}},
        DIVIDE('/'){int operate(int a,int b){return a/b;}};
        char character;
        Operation(char c){
            this.character=c;
        }
        public static Operation getOperation(String s){
            for(Operation operator:Operation.values()){
                if(operator.toString().equals(s))
                    return operator;
            }
            return null;
        }
        abstract int operate(int a,int b);
        @Override
        public String toString(){
            return ""+character;
        }
    }
    public int evalRPN(String[] tokens) {
        if(tokens==null) return -1;
        Stack<Integer> numbers=new Stack<Integer>();
        for(String token:tokens){
            Operation o=Operation.getOperation(token);
            if(o==null){
                numbers.push(Integer.parseInt(token));
            }
            else{
                int b=numbers.pop();
                int a=numbers.pop();
                int c=o.operate(a, b);
                numbers.push(c);
                
            }
        }
        return numbers.pop();
    }
}