leetcode--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

public class Solution {
public int evalRPN(String[] tokens) {
Stack<String> stack = new Stack<String>();
int num1 = 0;
int num2 = 0;
for(int i=0;i<tokens.length;i++){
switch (tokens[i]) {
case "+":
num1 = Integer.valueOf(stack.pop());
num2 = Integer.valueOf(stack.pop());
stack.add((num2+num1)+"");
break;
case "-":
num1 = Integer.valueOf(stack.pop());
num2 = Integer.valueOf(stack.pop());
stack.add((num2-num1)+"");
break;
case "*":
num1 = Integer.valueOf(stack.pop());
num2 = Integer.valueOf(stack.pop());
stack.add((num2*num1)+"");
break;
case "/":
num1 = Integer.valueOf(stack.pop());
num2 = Integer.valueOf(stack.pop());
stack.add((num2/num1)+"");
break;
default:
stack.add(tokens[i]);
break;
}
}
return Integer.valueOf(stack.pop());
}
}