leetcode 227. Basic Calculator II

class Solution {
public:
    int calculate(string s) {
        istringstream in('+' + s + '+'); // 这一步很好，简化了问题
        int ret = 0;
        int num;
        int partial = 0;
        char op;
        while(in>>op){
            if (op == '+' || op == '-'){
                ret += partial;
                in>>partial;
                partial *= (op == '+' ? 1 : -1);
            }
            else{
                in>>num;
                if (op == '*')
                    partial *= num;
                else
                    partial /= num;
            }
        }
        return ret;
    }
};