Language: C++

#include <iostream>
#include <set>

int main(){
  set <char> aph;
  string S;
  int flag = 1;
  cin >> S;
  for (int i = 0; i < S.length; i++)
    aph.insert(S[i]);
  for (char a ='a'; a <= 'z'; a++){
    if(!(aph.count(a))){
      flag = 0;
      cout << a << endl;
      break;
    }
  }
  if(flag)
    cout << "None" << endl;
}
  

Compiler output:
a.cc: In function 'int main()':
a.cc:5:3: error: 'set' was not declared in this scope
    5 |   set <char> aph;
      |   ^~~
a.cc:5:3: note: suggested alternatives:
In file included from /usr/include/c++/14/set:63,
                 from a.cc:2:
/usr/include/c++/14/bits/stl_set.h:96:11: note:   'std::set'
   96 |     class set
      |           ^~~
/usr/include/c++/14/set:87:13: note:   'std::pmr::set'
   87 |       using set = std::set<_Key, _Cmp, polymorphic_allocator<_Key>>;
      |             ^~~
a.cc:5:8: error: expected primary-expression before 'char'
    5 |   set <char> aph;
      |        ^~~~
a.cc:6:3: error: 'string' was not declared in this scope
    6 |   string S;
      |   ^~~~~~
a.cc:6:3: note: suggested alternatives:
In file included from /usr/include/c++/14/iosfwd:41,
                 from /usr/include/c++/14/ios:40,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/c++/14/bits/stringfwd.h:77:33: note:   'std::string'
   77 |   typedef basic_string<char>    string;
      |                                 ^~~~~~
In file included from /usr/include/c++/14/bits/locale_classes.h:40,
                 from /usr/include/c++/14/bits/ios_base.h:41,
                 from /usr/include/c++/14/ios:44:
/usr/include/c++/14/string:76:11: note:   'std::pmr::string'
   76 |     using string    = basic_string<char>;
      |           ^~~~~~
a.cc:8:3: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
    8 |   cin >> S;
      |   ^~~
      |   std::cin
/usr/include/c++/14/iostream:62:18: note: 'std::cin' declared here
   62 |   extern istream cin;           ///< Linked to standard input
      |                  ^~~
a.cc:8:10: error: 'S' was not declared in this scope
    8 |   cin >> S;
      |          ^
a.cc:10:5: error: 'aph' was not declared in this scope
   10 |     aph.insert(S[i]);
      |     ^~~
a.cc:12:10: error: 'aph' was not declared in this scope
   12 |     if(!(aph.count(a))){
      |          ^~~
a.cc:14:7: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   14 |       cout << a << endl;
      |       ^~~~
      |       std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:14:20: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   14 |       cout << a << endl;
      |                    ^~~~
      |                    std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
a.cc:19:5: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   19 |     cout << "None" << endl;
      |     ^~~~
      |     std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:19:23: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   19 |     cout << "None" << endl;
      |                       ^~~~
      |                       std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~