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());
    }
}