leetcode string to integer

class  {
public:
    int myAtoi(string str) {
        int n = str.length();
        int p = 0;
        bool sign = 0;
        int res = 0;
        //whitespace
        while(str[p] == ' ' && p < n) p++;
        //'+' and '-'
        if(str[p] == '+') {
            p++;
        }else if(str[p] == '-'){
            p++;
            sign = 1;
        }
        //digits
        for(; p < n; p++) {
            if(str[p] < '0' || str[p] > '9') break;
            if(res > INT_MAX / 10 || (res == INT_MAX / 10 && (str[p] - '0') > INT_MAX % 10)) {
                return sign == 1 ? INT_MIN : INT_MAX;
            }
            res = res * 10 + (str[p] - '0');
        }
        return sign == 1 ? (-1) * res : res;
    }
};