Reverse Polish expression (postfix expression)

package org.structure.stack;

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

/**
 * 逆波兰表达式 运算
 * @author cjj_1
 * @date 2020-08-07 13:59
 */
public class PolandNotation {
    
    
    public static void main(String[] args) {
    
    
        String suffixExpression = "3 4 + 5 * 6 -";
        String[] suffixArr = suffixExpression.split(" ");
        ArrayList<String> list = new ArrayList<String>();
        //将数组转换成 list
        for(int i =0 ;i< suffixArr.length;i++){
    
    
            list.add(suffixArr[i]);
        }
        //新建立一个栈
        Stack<Integer> stack = new Stack<Integer>();
         int num1 ;
         int num2;
         int res = -1  ;
        String item ;
        //遍历后波兰表达式
        for (int i=0;i<list.size();i++){
    
    
            item = list.get(i);
            if(item.matches("\\d+")){
    
    
                stack.push(Integer.valueOf(item));
            }else if (item.equals("+")){
    
    
                num2 = stack.pop();
                num1 = stack.pop();
                res = num1+num2;
            }else if(item.equals("-")){
    
    
                num1 = stack.pop();
                num2 = stack.pop();
                res = num1-num2;
            }else if(item.equals("*")) {
    
    
                num2 = stack.pop();
                num1 = stack.pop();
                res = num1 * num2;
            }else if(item.equals("/")) {
    
    
                num2 = stack.pop();
                num1 = stack.pop();
                res = num1 / num2;
            }else {
    
    
                throw  new RuntimeException("数字不对！");
            }
            if(res!=-1)
                stack.push(res);
        }
        System.out.println(stack.peek());
    }

}