逆波兰表达式（后缀表达式）

1、源码

PolandNotation类

package com.study.stack;
import java.util.ArrayList;
import java.util.List;
import java.util.Stack;
public class PolandNotation {
    public static void main(String[] args) {
        //定义一个逆波兰表达式
        //(3+4)*5-6   ->  3 4 + 5 * 6 -
        String suffixExpression = "3 4 + 5 * 6 -";
        List<String> listString = getListString(suffixExpression);
        System.out.println(listString);
        int res = calculate(listString);
        System.out.println(res);
    }
    public static List<String> getListString(String suffixExpression){
        String[] s = suffixExpression.split(" ");
        List<String> list = new ArrayList<>();
        for (String arr : s) {
            list.add(arr);
        }
        return list;
    }
    public static int calculate(List<String> ls){
        Stack<String> stack = new Stack<>();
        for (String l : ls) {
            if (l.matches("\\d+")){
                stack.push(l);
            }else {
                int num2 = Integer.parseInt(stack.pop());
                int num1 = Integer.parseInt(stack.pop());
                int res = 0;
                if(l.equals("+")){
                    res =num1+num2;
                }else if (l.equals("-")){
                    res = num1-num2;
                }else if (l.equals("*")){
                    res = num1*num2;
                }else if (l.equals("/")){
                    res = num1/num2;
                }else {
                    throw new RuntimeException("运算符有误");
                }
                stack.push(""+res);
            }
        }
        return Integer.parseInt(stack.pop());
    }
}

测试结果