["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

 1 public class Solution{
 2     public int evalRPN(String[] tokens) {
 3         Stack<Integer> nums = new Stack<Integer>();
 4         HashSet<String> opers = new HashSet<String>();
 5         opers.add("+");
 6         opers.add("-");
 7         opers.add("/");
 8         opers.add("*");
 9         int length = tokens.length;
10         for(int i = 0; i < length; ++i){
11             if(opers.contains(tokens[i])){
12                 int b = nums.pop();
13                 int a = nums.pop();
14                 nums.push(doOpers(a, b, tokens[i]));
15             }
16             else
17                 nums.push(Integer.parseInt(tokens[i]));
18         }
19         return nums.pop();
20     }
21 
22     //I used java 7.
23     public int doOpers(int a , int b, String opers){
24         int result = 0;
25         switch(opers){
26             case "+": result = a + b;
27                          break;
28             case "-": result = a - b;
29                          break;
30             case "*": result = a * b;
31                          break;
32             case "/": result = a / b;
33                          break;
34         }
35         return result;
36     }    
37 }