简单的atoi函数

int ui_atoi(const char *str)
{
    int sign = 1, num = 0;
    char *p = str;
    
    if (*p == '-')
    {
        sign = -1;
        p++;
    }
    
    while ((*p) != '\0')
    {
        if ((*p) >= '0' && (*p) <= '9')
        {
            num = num*10 + (*p-'0');
            p++;
        }
        else
            return -1;
    }
    
    return num*sign;
}