Evaluate Reverse Polish Notation

自己裸写，一遍过，牛！

搜了一下，别人跟我写的差不多，还有讲解： 戳参考链接

public class Solution {
    public int evalRPN(String[] tokens) {
        Stack<String> s = new Stack<String>();
        for(int i = 0; i < tokens.length; i++) {
            if(!isOperator(tokens[i])) {
                s.push(tokens[i]);
            } else {
                int first = Integer.parseInt(s.pop());
                int second = Integer.parseInt(s.pop());
                s.push(String.valueOf(compute(first, second, tokens[i])));
            }
        }
        return Integer.parseInt(s.pop());
    }
    
    private boolean isOperator(String s) {
        if(s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/")) return true;
        return false;
    }
    
    private int compute(int first, int second, String operator) {
        if(operator.equals("+")) return first + second;
        if(operator.equals("-")) return second - first;
        if(operator.equals("*")) return first * second;
        return second / first;
    }
}