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.

public class Solution {
    public int evalRPN(String[] tokens) {
        if(tokens == null || tokens.length == 0)
            return 0;
        
       Stack<Integer> stack = new Stack<Integer>();
       
       for(int i = 0; i< tokens.length; i++){
           String s = tokens[i];
           // couldn't use switch(), because in java switch is not available for String
           if(s.equals("+")){
               int num1 = stack.pop();
               int num2 = stack.pop();
               stack.push(num2 + num1);
           }else if(s.equals("-")){
               int num1 = stack.pop();
               int num2 = stack.pop();
               stack.push(num2 - num1);
           }else if(s.equals("*")){
               int num1 = stack.pop();
               int num2 = stack.pop();
               stack.push(num2 * num1);
           }else if(s.equals("/")){
               int num1 = stack.pop();
               int num2 = stack.pop();
               stack.push(num2 / num1);
           }else{
               stack.push(Integer.valueOf(s));
           }
       }
       
       return stack.pop();
    }
}