Language: C++

#include<iostream>
#include<vector>
#include<string>
#include<algorithm>

int main() {
	string s;
	cin >> s;

	if (s[0] == 'A') {
		cout << "T"<<endl;
	}
	else if(s[0]=='C'){
		cout<<"G"<<endl;
	
	}
	else if (s == "G") {
		cout << "C" << endl;
	}
	else {
		cout << "A" << endl;
	}
	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:7:9: error: 'string' was not declared in this scope
    7 |         string s;
      |         ^~~~~~
a.cc:7:9: 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:8:9: 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:8:16: error: 's' was not declared in this scope
    8 |         cin >> s;
      |                ^
a.cc:11:17: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   11 |                 cout << "T"<<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:11:30: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   11 |                 cout << "T"<<endl;
      |                              ^~~~
      |                              std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
a.cc:14:17: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   14 |                 cout<<"G"<<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:14:28: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   14 |                 cout<<"G"<<endl;
      |                            ^~~~
      |                            std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
a.cc:18:17: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   18 |                 cout << "C" << 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:18:32: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   18 |                 cout << "C" << endl;
      |                                ^~~~
      |                                std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
a.cc:21:17: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   21 |                 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:21:32: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   21 |                 cout << "A" << endl;
      |                                ^~~~
      |                                std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~