package cn.mrlij.stack;
 
import java.util.ArrayList;
import java.util.List;
import java.util.Stack;
 
/**
 * 逆波蘭表達(dá)式計(jì)算器實(shí)現(xiàn)
 */
public class PolandNotation {
 public static void main(String[] args) {
 //4*5-8+60+8/2
 String expression = "4 5 * 8 - 60 + 8 2 / +";
 List<String> list = getStrList(expression);
 System.out.println(list);
 //計(jì)算值，得結(jié)果
 int res = calc(list);
 System.out.println(res);
 }
 
 /**
 * 遍歷表達(dá)式，并將遍歷的結(jié)果放入list中
 * @param exp 表達(dá)式
 * @return
 */
 public static List<String> getStrList(String exp){
 String arr[] = exp.split(" ");//將字符串遍歷得到數(shù)組
 List<String> list = new ArrayList<>();
 for(String str : arr){
  list.add(str);
 }
 return list;
 }
 //計(jì)算表達(dá)式
 public static int calc(List<String> list ){
 //創(chuàng)建存放字符串的棧
 Stack<String> stack = new Stack<>();
 //遍歷list
 for (int i = 0;i<list.size();i++){
  //正則表達(dá)式匹配是否是數(shù)字
  if(list.get(i).matches("\\d+")){
  stack.push(list.get(i));//是數(shù)字則放入棧中
  }else {
  int num2 = Integer.parseInt(stack.pop());//彈出數(shù)字1
  int num1 = Integer.parseInt(stack.pop());//彈出數(shù)字2
  int res = 0;
  //進(jìn)行運(yùn)算
  if(list.get(i).equals("+")){
   res = num1 + num2;
  }else if(list.get(i).equals("-")){
   res = num1 - num2;
  }else if(list.get(i).equals("*")){
   res = num1 * num2;
  }else if(list.get(i).equals("/")){
   res = num1/num2;
  }else {
   throw new RuntimeException("不是操作符號(hào)！");
  }
  stack.push(""+res);
  }
 }
 //留在棧中的值就是最后的計(jì)算表達(dá)式結(jié)果
 return Integer.parseInt(stack.pop());
 }
}