Evaluate Reverse Polish Notation

 1 public class Solution {
 2     public int evalRPN(String[] tokens) {
 3         String operations ="+-*/";
 4         Stack<String> stack = new Stack<String>();
 5         for(String s:tokens){
 6             if(!operations.contains(s)){
 7                 stack.push(s);
 8             }
 9             else{
10                 int d1 = Integer.valueOf(stack.pop());
11                 int d2 = Integer.valueOf(stack.pop());
12                 int i = operations.indexOf(s);
13                 switch(i){
14                     case 0: stack.push(String.valueOf(d1+d2)); break;
15                     case 1: stack.push(String.valueOf(d2-d1)); break;
16                     case 2: stack.push(String.valueOf(d1*d2));break;
17                     case 3: stack.push(String.valueOf(d2/d1));break;
18                 }
19             }
20         }
21         return Integer.valueOf(stack.pop());
22     }
23 }