題目：字尾表示式求和

思路：遇見數字直接出棧，遇見運算子號，用異常代替判斷。處理異常時出兩個元素，進行運算並將結果入棧；

import java.util.Stack;
public class Solution {
    public int evalRPN(String[] tokens) {
        Stack<Integer> stack = new Stack<Integer>();
        for(int i = 0;i<tokens.length;i++){
            try{
                int num = Integer.parseInt(tokens[i]);
                stack.add(num);
            }catch (Exception e) {
                int b = stack.pop();
                int a = stack.pop();
                stack.add(get(a, b, tokens[i]));
            }
        }
        return stack.pop();
    }
    private int get(int a,int b,String operator){
        switch (operator) {
        case "+":
            return a+b;
        case "-":
            return a-b;
        case "*":
            return a*b;
        case "/":
            return a/b;
        default:
            return 0;
        }
    }
}