Language: C++

using System;
using System.Collections.Generic;
using System.Linq;

class Procon
{
    public Procon() { }
    static void Main(string[] agrs)
    {
        new Procon().Do();
    }

    Scanner cin;

    public void Do()
    {
        cin = new Scanner();
        string b = cin.nextString();
        if (b == "A")
        {
            Console.WriteLine("T");
        }
        if (b == "T")
        {
            Console.WriteLine("A");
        }
        if (b == "C")
        {
            Console.WriteLine("G");
        }
        if (b == "G")
        {
            Console.WriteLine("C");
        }
    }
}

class Scanner
{
    string[] s;
    int i;
    int max_i;

    public Scanner()
    {
        s = new string[0];
        max_i = 0;
        i = 0;
    }

    public string nextString()
    {
        if (i >= s.Length)
        {
            s = Console.ReadLine().Split(' ');
            max_i = s.Length;
            i = 0;
            if (max_i == 0)
            {
                return "";
            }
            return s[i++];
        }
        else
        {
            return s[i++];
        }
    }

    public int nextInt()
    {
        return int.Parse(nextString());
    }

    public long nextLong()
    {
        return long.Parse(nextString());
    }

    public double nextDouble()
    {
        return double.Parse(nextString());
    }

    public string[] arrayString()
    {
        return Console.ReadLine().Split(' ');
    }

    public int[] arrayInt()
    {
        string[] str = Console.ReadLine().Split(' ');
        int[] array = new int[str.Length];
        for (int i = 0; i < str.Length; i++)
        {
            array[i] = int.Parse(str[i]);
        }
        return array;
    }

    public long[] arrayLong()
    {
        string[] str = Console.ReadLine().Split(' ');
        long[] array = new long[str.Length];
        for (int i = 0; i < str.Length; i++)
        {
            array[i] = long.Parse(str[i]);
        }
        return array;
    }

    public double[] arrayDouble()
    {
        string[] str = Console.ReadLine().Split(' ');
        double[] array = new double[str.Length];
        for (int i = 0; i < str.Length; i++)
        {
            array[i] = double.Parse(str[i]);
        }
        return array;
    }
}

Compiler output:
a.cc:1:7: error: expected nested-name-specifier before 'System'
    1 | using System;
      |       ^~~~~~
a.cc:2:7: error: expected nested-name-specifier before 'System'
    2 | using System.Collections.Generic;
      |       ^~~~~~
a.cc:3:7: error: expected nested-name-specifier before 'System'
    3 | using System.Linq;
      |       ^~~~~~
a.cc:7:11: error: expected ':' before 'Procon'
    7 |     public Procon() { }
      |           ^~~~~~~
      |           :
a.cc:8:22: error: 'string' has not been declared
    8 |     static void Main(string[] agrs)
      |                      ^~~~~~
a.cc:8:31: error: expected ',' or '...' before 'agrs'
    8 |     static void Main(string[] agrs)
      |                               ^~~~
a.cc:13:5: error: 'Scanner' does not name a type
   13 |     Scanner cin;
      |     ^~~~~~~
a.cc:15:11: error: expected ':' before 'void'
   15 |     public void Do()
      |           ^~~~~
      |           :
a.cc:36:2: error: expected ';' after class definition
   36 | }
      |  ^
      |  ;
a.cc: In static member function 'static void Procon::Main(int*)':
a.cc:10:21: error: expected ';' before '.' token
   10 |         new Procon().Do();
      |                     ^
a.cc: In member function 'void Procon::Do()':
a.cc:17:9: error: 'cin' was not declared in this scope
   17 |         cin = new Scanner();
      |         ^~~
a.cc:17:19: error: expected type-specifier before 'Scanner'
   17 |         cin = new Scanner();
      |                   ^~~~~~~
a.cc:18:9: error: 'string' was not declared in this scope
   18 |         string b = cin.nextString();
      |         ^~~~~~
a.cc:19:13: error: 'b' was not declared in this scope
   19 |         if (b == "A")
      |             ^
a.cc:21:13: error: 'Console' was not declared in this scope
   21 |             Console.WriteLine("T");
      |             ^~~~~~~
a.cc:23:13: error: 'b' was not declared in this scope
   23 |         if (b == "T")
      |             ^
a.cc:25:13: error: 'Console' was not declared in this scope
   25 |             Console.WriteLine("A");
      |             ^~~~~~~
a.cc:27:13: error: 'b' was not declared in this scope
   27 |         if (b == "C")
      |             ^
a.cc:29:13: error: 'Console' was not declared in this scope
   29 |             Console.WriteLine("G");
      |             ^~~~~~~
a.cc:31:13: error: 'b' was not declared in this scope
   31 |         if (b == "G")
      |             ^
a.cc:33:13: error: 'Console' was not declared in this scope
   33 |             Console.WriteLine("C");
      |             ^~~~~~~
a.cc: At global scope:
a.cc:40:5: error: 'string' does not name a type
   40 |     string[] s;
      |     ^~~~~~
a.cc:44:11: error: expected ':' before 'Scanner'
   44 |     public Scanner()
      |           ^~~~~~~~
      |           :
a.cc:51:11: error: expected ':' before 'string'
   51 |     public string nextString()
      |           ^~~~~~~
      |           :
a.cc:51:12: error: 'string' does not name a type
   51 |     public string nextString()
      |            ^~~~~~
a.cc:70:11: error: expected ':' before 'int'
   70 |     public int nextInt()
      |           ^~~~
      |           :
a.cc:75:11: error: expected ':' before 'long'
   75 |     public long nextLong()
      |           ^~~~~
      |           :
a.cc:80:11: error: expected ':' before 'double'
   80 |     public double nextDouble()
      |           ^~~~~~~
      |           :
a.cc:85:11: error: expected ':' before 'string'
   85 |     public string[] arrayString()
      |           ^~~~~~~
      |           :
a.cc:85:12: error: 'string' does not name a type
   85 |     public string[] arrayString()
      |            ^~~~~~
a.cc:90:11: error: expected ':' before 'int'
   90 |     public int[] arrayInt()
      |           ^~~~
      |           :
a.cc:90:15: error: expected unqualified-id before '[' token
   90 |     public int[] arrayInt()
      |               ^
a.cc:101:11: error: expected ':' before 'long'
  101 |     public long[] arrayLong()
      |           ^~~~~
      |           :
a.cc:101:16: error: expected unqualified-id before '[' token
  101 |     public long[] arrayLong()
      |                ^
a.cc:112:11: error: expected ':' before 'double'
  112 |     public double[] arrayDouble()
      |           ^~~~~~~
      |           :
a.cc:112:18: error: expected unqualified-id before '[' token
  112 |     public double[] arrayDouble()
      |                  ^
a.cc:122:2: error: expected ';' after class definition
  122 | }
      |  ^
      |  ;
a.cc: In constructor 'Scanner::Scanner()':
a.cc:46:9: error: 's' was not declared in this scope
   46 |         s = new string[0];
      |         ^
a.cc:46:17: error: 'string' does not name a type
   46 |         s = new string[0];
      |                 ^~~~~~
a.cc: In member function 'int Scanner::nextInt()':
a.cc:72:16: error: expected primary-expression before 'int'
   72 |         return int.Parse(nextString());
      |                ^~~
a.cc:72:16: error: expected ';' before 'int'
a.cc:72:19: error: expected unqualified-id before '.' token
   72 |         return int.Parse(nextString());
      |                   ^
a.cc: In member function 'long int Scanner::nextLong()':
a.cc:77:16: error: expected primary-expression before 'long'
   77 |         return long.Parse(nextString());
      |                ^~~~
a.cc:77:16: error: expected ';' before 'long'
a.cc:77:20: error: expected unqualified-id before '.' token
   77 |         return long.Parse(nextString());
      |                    ^
a.cc: In member function 'double Scanner::nextDouble()':
a.cc:82:16: error: expected primary-expression before 'double'
   82 |         return double.Parse(nextString());
      |                ^~~~~~
a.cc:82:16: error: expected ';' before 'double'
a.cc:82:22: error: expected unqualified-id before '.' token
   82 |         return double.Parse(nextString());
      |                      ^