检测是否是全数字

bool isDigitStr(const char* cstr){    if (NULL == cstr || cstr[0] == 0)    {        return false;    }    int len = strlen(cstr);    int pos = 0;    if (cstr[0] == '-' || cstr[0] == '+')    {        if (len <= 1)        {            return false;        }        pos++;    }    while (pos < len)    {        if (cstr[pos] < '0' || cstr[pos] > '9')        {            return false;        }        pos++;    }    return true;}