数据结构之栈对逆BoLand表达式的计算

package com.ebiz.stack;

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

/**
 * @author YHj
 * @create 2019-07-24 14:17
 * 逆波兰表达式,实现计算器
 */
public class NiPoLandCalculator {

    public static void main(String[] args) {
        //定义一个逆波兰表达式(后缀表达式)  (30+4)*5-6 => 30 4+5*6-
        //为了方便,数字以及运算符之间用空格隔开
        String expression="30 4 + 5 * 6 - ";

        List<String> list = getList(expression);

        int resul=getResult(list);
        System.out.println("resul = " + resul);

    }

    //输出计算结果
    private static int getResult(List<String> list) {
        Stack<String> stack = new Stack<>();
        for (String s : list) {
            //匹配数字,可匹配多位数
            if (s.matches("\\d+")){
                stack.push(s);
            }else {
                int num01=Integer.parseInt(stack.pop());
                int num02=Integer.parseInt(stack.pop());
                int result=0;
                if (s.equals("+")){
                    result=num01+num02;
                }else if (s.equals("-")){
                    result=num02-num01;
                }else if (s.equals("*")){
                    result=num02*num01;
                }else if (s.equals("/")){
                    result=num02/num01;
                }else {
                    throw new RuntimeException("无法解析的字符串");
                }
                stack.push(""+result);
            }
        }
        return Integer.parseInt(stack.pop());
    }

    //将表达式的值以空格进行拆分,加入到arraylist中,便于操作(不适用索引)
    private static List<String> getList(String str) {
        String[] split = str.split(" ");
        ArrayList<String> list = new ArrayList<>();
        for (String s : split) {
            list.add(s);
        }
        return list;
    }


}