인프런 - 후위 연산자(postpix)

import java.util.*;
class Main {	
	public int solution(String str){
		int answer=0;
        Stack<Integer> stack = new Stack<>();
        for(char x : str.toCharArray()) {
            if(Character.isDigit(x)) stack.push(x-48); // 48 ='0' '5'-48 -> 숫자 5가 됨
            else{
                //숫자 두개꺼내기. 항상 lt 연산자 rt로 연산해야함
                int rt = stack.pop();
                int lt = stack.pop();
                if(x == '+') stack.push(lt+rt);
                else if(x == '-') stack.push(lt-rt);
                else if(x == '*') stack.push(lt*rt);
                else if(x == '/') stack.push(lt/rt);
            }
        }
        answer = stack.get(0);
    
        
		return answer;
	}
	public static void main(String[] args){
		Main T = new Main();
		Scanner kb = new Scanner(System.in);
		String str=kb.next();
		System.out.println(T.solution(str));
	}
}