package leetcode练习题.每日一题;
import java.util.Set;
import java.util.Stack;
public class 逆波兰表达式求值 {
    //声明一个栈保存整型，声明一个Set集合保存算数运算符
    static Stack<Integer> stk = new Stack<Integer>();
    static Set<String> s = Set.of("+","-","*","/");
    public static void eval(String s){
        int b = stk.peek();stk.pop();
        int a = stk.peek();stk.pop();
        if(s.equals("+")) stk.push(a+b);
        else if(s.equals("-")) stk.push(a-b);
        else if(s.equals("*")) stk.push(a*b);
        else{
            stk.push(a/b);
        }
    }
    public static int evalRPN(String[] tokens) {
        for (String t: tokens) {
            //如果是数字，转换为整型保存到栈中---isDigit(char ch)
            //因为数据中有两位数以及负数，且非数字的长度都是1
            //所以需要特判一下
            if(Character.isDigit(t.charAt(0)) || t.length() >1)  stk.push(Integer.parseInt(t));
            //不是整型则进行运算
            else eval(t);
        }
        return stk.peek();
    }

    public static void main(String[] args) {
        String[] tokens = {"10","6","9","3","+","-11","*","/","*","17","+","5","+"};
        System.out.println(evalRPN(tokens));

    }
}