String to Integer (atoi)

class Solution {public:        int atoi(const char *str) {                // Start typing your C/C++ solution below                // DO NOT write int main() function                if (str == NULL) {                        return 0;                }                int flag = 0;                long long ret;                while (*str == ' ') {                        str++;                }                if (str[0] == '-') {                        flag = 1;                        str++;                } else if (str[0] == '+') {                        flag = 0;                        str++;                }                ret = 0;                while (*str) {                                        if (*str<'0' || *str >'9') {                                return ret;                        }                        ret *= 10;                        if (flag) {                                ret -= *str - '0';                        } else {                                ret += *str - '0';                        }                        if (ret > INT_MAX && flag == 0) {                                return  (int)INT_MAX;                        }                        if (ret < INT_MIN && flag == 1) {                                return  (int)INT_MIN;                        }                        str++;                }                return ret;        }};