例如: (3+4)X5-6对应的后缀表达式就是3 4 + 5 X 6-，针对后缀表达式求值步骤如下:
//        1.从左至右扫描，将3和4压入堆栈;
//        2.遇到+运算符，因此弹出4和3 (4 为栈顶元素，3为次顶元素)，计算出3+4的值，得7，再将7入栈;
//        3.将5入栈;
//        4.接下来是X运算符，因此弹出5和7,计算出7X5=35,将35入栈;
//        5.将6入栈;
//        6.最后是-运算符，计算出35-6的值，即29，由此得出最终结果

import java.util.Stack;
public class LatterException {
    public static void main(String[] args) {
//        char ch[]={'0','1','2'};
//        System.out.println(new String(ch,0,1));
        LatterException latterException = new LatterException();
        System.out.println(latterException.cal1("3 4 + 5 * 6 -"));
    }
    public double cal1(String exception){
        String[] arr = exception.split(" ");
        print(arr);
        System.out.println(arr);
//      1、创建存放数字的栈
        Stack<Double> doubleStack = new Stack<>();
        int pos=0,len=arr.length;
        while(pos<len){
            if(isOperator(arr[pos])){
                double num1=doubleStack.pop();
                double num2=doubleStack.pop();
                doubleStack.push(cal(arr[pos],num2,num1));
                System.out.println(doubleStack);
            }else{
                doubleStack.push(Double.parseDouble(arr[pos]));
                System.out.println(doubleStack);
            }
            pos++;
        }
        return doubleStack.pop();
    }
    public boolean isOperator(String oper){
        //用String类的equals()方法，来判断而且形式为"+".equals(oper)以防止空对象的出现,如果用oper=="-"，那么返回值一直是false
        if("+".equals(oper)||"-".equals(oper)||"*".equals(oper)||"/".equals(oper)){
            return true;
        }
        return false;
    }
    //计算函数
    public  double cal(String oper,double num1,double num2) {
        return switch (oper) {
            case "+" -> num1 + num2;
            case "-" -> num1 - num2;
            case "*" -> num1 * num2;
            case "/" -> num1 / num2;
            default -> throw new RuntimeException("运算符有错误");
        };
    }
    public void  print(String arr[]){
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i]+" ");
        }
        System.out.println();
    }
}