Lintcode 424. Evaluate Reverse Polish Notation

public class Solution {

    private static final String[] ops = {"+", "-", "*", "/"};

    /**
     *
     * T: O(n), S: O(n)
     *
     * @param tokens: The Reverse Polish Notation
     * @return: the value
     */
    public int evalRPN(String[] tokens) {
        Stack<Integer> stack = new Stack<Integer>();

        for(String token : tokens) {
            if(isOp(token)) {
                int num2 = stack.pop();
                int num1 = stack.pop();
                String op = token;
                stack.push(eval(num1, num2, op));
            } else {
                stack.push(new Integer(token));
            }
        }
        return stack.peek();
    }

    private int eval(int num1, int num2, String op) {
        switch(op) {
            case "+":
                return num1 + num2;
            case "-":
                return num1 - num2;
            case "*":
                return num1 * num2;
            case "/":
                return num1 / num2;
        }
        throw new IllegalArgumentException();
    }

    private boolean isOp(String token) {
        for(String op : ops) {
            if(op.equals(token)) {
                return true;
            }
        }
        return false;
    }
}