面试：atoi和itoa的实现

1.int atoi(const char* src)

• nullptr指针
• 空白字符' ','\t','\n'
• 符号位
• 避免值溢出
• 出错信息保存在全局变脸errnum中

 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;
 }

2. char* itoa(int val,char* buf,size_t radix)

• 指针有效性判断
• 符号位
• 基数（10,16,2进制）
• reverse操作

 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;
 }