数据超过时要及时返回，不然容易出错

class Solution {
public:
    int strToInt(string str) {
        int i=0;
        long long res=0;
        while(str[i]==' ')i++;
         int rt=1;
         if(str[i]=='+'){i++;}
         else if(str[i]=='-'){rt=-rt;i++;}
        while(str[i]>='0'&&str[i]<='9'){
            res=res*10+str[i]-'0';
            i++;
        if(res*rt>INT_MAX)return INT_MAX;
        if(res*rt<INT_MIN)return INT_MIN;
        }

        return res*rt;
    }
};