字符串转换成数字

class Solution {
public:
    int myAtoi(string str) {
        long num = 0;
        int sign = 1;
        //int i = str.find_first_not_of(' ');
        //sign = (str[i++] == '-') ? -1 : 1;
        int i = 0;
        i = str.find_first_not_of(' ');
        if(str[i] == '+' || str[i] == '-')
            sign = (str[i++] == '-') ? -1:1;
        if(str[i] < '0' || str[i] > '9')
            return 0;
        while(str[i] >= '0' && str[i] <= '9' && i < str.size()){
                num = num*10 + (str[i++] - '0');
                if(sign*num > INT_MAX) return INT_MAX;
                if(sign*num < INT_MIN) return INT_MIN;
        }  
        return sign*num;
    }
};