Language: C++

#include<bits/stdc++.h>
using UL = unsigned;
using LL = long long;
using ULL = unsigned long long;
#define rep(i, n) for(int i = 0; i < (n); i++)

void Solve(){
  char c; cin >> c;
  if(c == 'A') cout << "T" << endl;
  if(c == 'T') cout << "A" << endl;
  if(c == 'G') cout << "C" << endl;
  if(c == 'C') cout << "G" << endl;
}
int main(){
  Solve(); return 0;
}

Compiler output:
a.cc: In function 'void Solve()':
a.cc:8:11: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
    8 |   char c; cin >> c;
      |           ^~~
      |           std::cin
In file included from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:146,
                 from a.cc:1:
/usr/include/c++/14/iostream:62:18: note: 'std::cin' declared here
   62 |   extern istream cin;           ///< Linked to standard input
      |                  ^~~
a.cc:9:16: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
    9 |   if(c == 'A') 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:9:31: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
    9 |   if(c == 'A') cout << "T" << endl;
      |                               ^~~~
      |                               std::endl
In file included from /usr/include/c++/14/istream:41,
                 from /usr/include/c++/14/sstream:40,
                 from /usr/include/c++/14/complex:45,
                 from /usr/include/c++/14/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:127:
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
a.cc:10:16: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   10 |   if(c == 'T') 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:10:31: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   10 |   if(c == 'T') 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:11:16: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   11 |   if(c == 'G') 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:11:31: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   11 |   if(c == 'G') 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:12:16: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   12 |   if(c == 'C') 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:12:31: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   12 |   if(c == 'C') cout << "G" << endl;
      |                               ^~~~
      |                               std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~