[编程题] 奇怪的表达式求值代码

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        while(in.hasNext()) {
            String s = in.next();
            solve(s);
        }
        
    }
    
    private static void solve(String s) {
        
        String operators = "+-*";
        
        Stack<Integer> stack = new Stack<>();
        Stack<Character> os = new Stack<>();
        
        for(int i=0;i<s.length();i++) {
            if(s.charAt(i)>='0' && s.charAt(i)<='9') {
                stack.push(Integer.valueOf(s.charAt(i)+""));
                if(stack.size() == 2) {
                    int a = stack.pop();
                    int b =stack.pop();
                    if(os.peek() == '+') {
                        stack.push(a+b);
                    }
                    else if(os.peek() == '-')
                        stack.push(b-a);
                    else if(os.peek() == '*')
                        stack.push(b*a);
                }
            }
            else {
                os.push(s.charAt(i));
            }
        }
        System.out.println(stack.pop());
    }
}