BM49 表达式求值

import java.util.*;
public class Solution {
    public ArrayList<Integer> function(String s, int index){
        Stack<Integer> stack = new Stack<Integer>(); 
        int num = 0;
        char op = '+';
        int i;
        for(i = index; i < s.length(); i++){
            if(s.charAt(i) >= '0' && s.charAt(i) <= '9'){ 
                num = num * 10 + s.charAt(i) - '0';
                if(i != s.length() - 1)
                    continue;
            }
            if(s.charAt(i) == '('){
                ArrayList<Integer> res = function(s, i + 1);
                num = res.get(0);
                i = res.get(1);
                if(i != s.length() - 1)
                    continue;
            }            
            switch(op){
            case '+': 
                stack.push(num);
                break;
            case '-':
                stack.push(-num);
                break;
            case '*':  
                int temp = stack.pop();
                stack.push(temp * num);
                break;
            }
            num = 0;
            if(s.charAt(i) == ')') 
                break; 
            else 
                op = s.charAt(i);
        }
        int sum = 0;
        while(!stack.isEmpty())  
            sum += stack.pop();
        ArrayList<Integer> temp = new ArrayList<Integer>();
        temp.add(sum);
        temp.add(i);
        return temp; 
    }
    public int solve (String s) {
        ArrayList<Integer> res = function(s, 0);
        return res.get(0);
    }
}