Hexadecimal to Decimal Using Recursion C++

Problem Statment: Write a (regular or tail) recursive function that converts a valid hexadecimal string into a decimal value no longer than 64 bits.

"0x0" => 0
"0x1" => 1
"0xABCD" => 43981

int charTodec(char const ch){

  if(ch>='a'&& ch<='f')
    return ch-'a'+10;
  if(ch>='A'&& ch<='F')
    return ch-'A'+10;
  else
  return ch -'0';
}

unsigned long long int hexTodec(char const* str, unsigned long long result){

  if(*str == '\0')
    return result;
  else{
    result=result*16+charTodec(*str);
    return hexTodec(str+1,result);
  }
}

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