逆波兰表达式计算

public class  Polanexpr{
    public static void main(String[] args) {
        String expr = "3 4 + 5 * 6 - "; //逆波兰表达式 (3+4）*5-6
        List<String> listString = getListString(expr); //将逆波兰表达式转换为List
        int res = cale(listString);
        System.out.println(res);

    }
        //将逆波兰表达式转换为list
    public static List<String> getListString(String expr){
        //将表达式分割成数组
        String[] split = expr.split(" ");
        List<String> list = new ArrayList<String>();
        for(String ele: split){
            list.add(ele);
        }
        return list;

    }
    //表达式计算
    public static int cale(List<String> ls){
        Stack<String> stack = new Stack<String>();
        //遍历 ls
        for(String item: ls){
            //使用正则匹配数值
            if(item.matches("\\d+")){
                //数值直接入栈
                stack.push(item);
            }else{
                //如果是运算符则需在栈中弹出两个数，进行运算
                int num1 = Integer.parseInt(stack.pop());
                int num2 = Integer.parseInt(stack.pop());
                int res = 0;
                switch (item){
                    case "+":
                        res = num1 + num2;
                        break;
                    case "-":
                        res = num2 - num1;
                        break;
                    case "*":
                        res = num1 * num2;
                        break;
                    case "/":
                        res = num2 / num1;
                        break;
                    default:
                        break;
                }
                //运算的结果压入栈中
                stack.push(res+"");
            }
        }
        //最后留在栈中的元素即为结果
        return Integer.parseInt(stack.pop());
    }
}