自己动手写C语言库函数（6） atoi

int atoi(const char* str) 
{
    int num = 0;
    int sign = 1;
    const int len = strlen(str);
    int i = 0;
    while (str[i] == ' ' && i < len) i++;
    if (str[i] == '+') i++;
    if (str[i] == '-') 
    {
        sign = -1;
        i++;
    }
    for (; i < len; i++) 
    {
        if (str[i] < '0' || str[i] > '9')
            break;
        if (num > INT_MAX / 10 || (num == INT_MAX / 10 && (str[i] - '0') > INT_MAX % 10)) 
        {
            return sign == -1 ? INT_MIN : INT_MAX;
        }
        num = num * 10 + str[i] - '0';
    }
    return num * sign;
}