Evaluate Reverse Polish Notation

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Some examples:

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

The concept is:
When meeting the number, push into the stack.
When meeting the operator, pop the top 2 number and compute the value, then push the result back into the stack.
Until the end of the expression.
Output the top (last) value in the stack.

 1 public class Solution {
 2     public int evalRPN(String[] tokens) {
 3         if(tokens == null || tokens.length == 0)
 4             return 0;
 5         
 6        Stack<Integer> stack = new Stack<Integer>();
 7        
 8        for(int i = 0; i< tokens.length; i++){
 9            String s = tokens[i];
10            // couldn't use switch(), because in java switch is not available for String
11            if(s.equals("+")){
12                int num1 = stack.pop();
13                int num2 = stack.pop();
14                stack.push(num2 + num1);
15            }else if(s.equals("-")){
16                int num1 = stack.pop();
17                int num2 = stack.pop();
18                stack.push(num2 - num1);
19            }else if(s.equals("*")){
20                int num1 = stack.pop();
21                int num2 = stack.pop();
22                stack.push(num2 * num1);
23            }else if(s.equals("/")){
24                int num1 = stack.pop();
25                int num2 = stack.pop();
26                stack.push(num2 / num1);
27            }else{
28                stack.push(Integer.valueOf(s));
29            }
30        }
31        
32        return stack.pop();
33     }
34 }