int errnum = ;

 int atoi(const char* src){

     if(src == nullptr) {

         errnum = -; //empty string

         return ;

     }

     //remove whitespace characters.

     while(*src == ' ' || *src == '\t' || *src == '\n'){

         src++;

     }

     int sign = ;

     if(*src == '+'){

         src++;

     }else if(*src == '-'){

         sign = -;

         src++;

     }

     //only with sign bit

     if(*src == '\0') {

         errnum = -;

         return ;

     }

     long long res = ;

     while(*src != '\0'){

         if(*src >= '' && *src <= ''){

             res = res *  + *src - '';

             if((sign ==  && res > 0x7fffffff) || (sign == - && (-*res) < (int)0x80000000)){

                 errnum = -;  //out of range

                 return ;

             }

         }else{

             errnum = -;  //illegal character

             return ;

         }

         src++;

     }

     return sign*res;

 }

 char* itoa(int val,char* buf,size_t radix){

     assert(buf != nullptr);

     char* p = buf;

     if(val < ){

         *p++ = '-';

         val = - * val;

     }

     int a = ;

     do{

         a = val % radix;

         val /= radix;

         if(a > ){

             *p++ = char(a-+'a');

         }else{

             *p++ = (char)(a + '');

         }

     }while(val != );

     *p = '\0';

     //reverse

     size_t len = strlen(buf);

     int left = ;

     if(*buf == '-' || *buf == '+'){

         left++;

     }

     int right = len - ;

     while(left < right){

         swap(buf[left++],buf[right--]);

     }

     return buf;

 }