basic calculator

class Solution {
public:
    int (string s) {
        stack<int> stk;
        char sign = '+';
        int tmp = 0;
        int res = 0;
        for (int i = 0; i < s.size(); i++){
            if (isdigit(s[i]))
                tmp = tmp * 10 + s[i] - '0';
            if (!isdigit(s[i]) && !isspace(s[i]) || i == s.size() - 1){
                if (sign == '+')
                    stk.push(tmp);
                else if (sign == '-')
                    stk.push(-tmp);
                else{
                    int num;
                    if (sign == '*')
                        num = stk.top() * tmp;
                    else
                        num = stk.top() / tmp;
                    stk.pop();
                    stk.push(num);
                }
                sign = s[i];
                tmp = 0;
            }
        }
        while(!stk.empty()){
            res += stk.top();
            stk.pop();
        }  
        return res;
        
    }
};