Java實現簡單的表達式計算

//不支持單目運算符，沒有用逆波蘭什麼的那些概念，就是簡單的入棧和出棧操作
import java.util.Scanner;
import java.util.Stack;
@SuppressWarnings("rawtypes")
public class Main{
    
    static Stack tmpStk=new Stack();
    static Stack opStk=new Stack();
    
    //定義運算符優先級
    public static int leavel(char s){
        switch(s){
        case '(': return 0;
        case '+': 
        case '-': return 1;
        case '*':
        case '/': return 2;
        }
        return -1;
    }
    public static int calc(int val1,int val2,char c){
        switch(c){
        case '+': return val1+val2;
        case '-': return val2-val1;
        case '*': return val1*val2;
            }
            if(val1==0) return 0;
            return val1/val2;
        }

    @SuppressWarnings("unchecked")
    public static void main(String args[]){
    Scanner input=new Scanner(System.in);
        String s=input.nextLine();    
        input.close();
        char a[]=s.toCharArray();
        boolean flg=false;
        
        for(int i=0;i<s.length();i++)
        {
            flg=false;
            int x=0;
            char c=a[i];
            
            while(i<a.length & c<='9' & c>='0')
            {
                x=x*10+(c-'0');
                i++;
                if(i<a.length)c=a[i];
                flg=true;
            }
            if(flg)
                tmpStk.push(x);
            if(c>'9' || c<'0')
            {
                if(opStk.isEmpty() || c=='(') opStk.push(c);
                else if(c==')')
                {
                    while((char)opStk.peek()!='(')
                    {
                        int t=calc((int)tmpStk.pop(),(int)tmpStk.pop(),(char)opStk.pop());
                        tmpStk.push(t);
                    }
                    opStk.pop();
                }
                else if(leavel(c)<=leavel((char)opStk.peek()))
                        {
                            int t=calc((int)tmpStk.pop(),(int)tmpStk.pop(),(char)opStk.pop());
                            tmpStk.push(t);
                            opStk.push(c);
                        }
                else    opStk.push(c);
            }
        }
        while(!opStk.isEmpty())
        {
            int aa=(int)tmpStk.pop();
            int b=(int)tmpStk.pop();
            char cc=(char)opStk.pop();
            int t=calc(aa,b,cc);
            tmpStk.push(t);
        }
        System.out.println(tmpStk.pop());
    }
}