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这道题用Stack of Integer要方便得多，用Stack of string则要来回转换。

class Solution {
    public int evalRPN(String[] tokens) {
        Stack<Integer> stack = new Stack<>();
        for (String token : tokens){
            if (token.equals("+")){
                int num1 = stack.pop();
                int num2 = stack.pop();
                int sum = num1 + num2;
                stack.push(sum);
            } else if (token.equals("-")){
                int num1 = stack.pop();
                int num2 = stack.pop();
                int subs = num2 - num1;
                stack.push(subs);
            } else if (token.equals("*")){
                int num1 = stack.pop();
                int num2 = stack.pop();
                int prod = num1 * num2;
                stack.push(prod);
            } else if (token.equals("/")){
                int num1 = stack.pop();
                int num2 = stack.pop();
                int quot = num2 / num1;
                stack.push(quot);
            } else {
                int num = Integer.parseInt(token);
                stack.push(num);
            }
        }
        return stack.pop();
    }
}