Task: 21 game
C: 



#define _CRT_SECURE_NO_WARNINGS

#include <stdio.h>
#include <stdlib.h>
#include <time.h>


#if __STDC_VERSION__ >= 199901L || _MSC_VER >= 1800
#include <stdbool.h>
#else
#define bool int
#define true  1
#define false 0
#endif

#define GOAL               21
#define NUMBER_OF_PLAYERS   2
#define MIN_MOVE            1
#define MAX_MOVE            3
#define BUFFER_SIZE       256

#define _(STRING) STRING



static char DESCRIPTION[] = 
    "21 Game                                                          \n"
    "                                                                 \n"
    "21 is a two player game, the game is played by choosing a number \n"
    "(1, 2, or 3) to be added to the running total. The game is won by\n"
    "the player whose chosen number causes the running total to reach \n"
    "exactly 21. The running total starts at zero.                    \n\n";

static int total;


void update(char* player, int move)
{
    printf("%8s:  %d = %d + %d\n\n", player, total + move, total, move);
    total += move;
    if (total == GOAL)
        printf(_("The winner is %s.\n\n"), player);
}


int ai()
{

#if GOAL < 32 && MIN_MOVE == 1 && MAX_MOVE == 3
    static const int precomputed[] = { 1, 1, 3, 2, 1, 1, 3, 2, 1, 1, 3, 2, 1, 1,
        3, 2, 1, 1, 3, 2, 1, 1, 3, 2, 1, 1, 3, 2, 1, 1, 3 };
    update(_("ai"), precomputed[total]);
#elif MIN_MOVE == 1 && MAX_MOVE == 3
    static const int precomputed[] = { 1, 1, 3, 2};
    update(_("ai"), precomputed[total % (MAX_MOVE + 1)]);
#else
    int i;
    int move = 1;
    for (i = MIN_MOVE; i <= MAX_MOVE; i++)
        if ((total + i - 1) % (MAX_MOVE + 1) == 0)
            move = i;
    for (i = MIN_MOVE; i <= MAX_MOVE; i++)
        if (total + i == GOAL)
            move = i;
    update(_("ai"), move);
#endif
}


void human(void)
{
    char buffer[BUFFER_SIZE];
    int move;
 
    while ( printf(_("enter your move to play (or enter 0 to exit game): ")),
            fgets(buffer, BUFFER_SIZE, stdin), 
            sscanf(buffer, "%d", &move) != 1 ||
            (move && (move < MIN_MOVE || move > MAX_MOVE || total+move > GOAL)))
        puts(_("\nYour answer is not a valid choice.\n"));
    putchar('\n');
    if (!move) exit(EXIT_SUCCESS);
    update(_("human"), move);
}


int main(int argc, char* argv[])
{
    srand(time(NULL));
    puts(_(DESCRIPTION));
    while (true)
    {
        puts(_("\n---- NEW GAME ----\n"));
        puts(_("\nThe running total is currently zero.\n"));
        total = 0;

        if (rand() % NUMBER_OF_PLAYERS)
        {
            puts(_("The first move is AI move.\n"));
            ai();
        }
        else
            puts(_("The first move is human move.\n"));

        while (total < GOAL)
        {
            human();
            ai();
        }
    }
}

C#: 

using System;

namespace _21Game
{
    public class Program
    {
        private const string computerPlayer = "Computer";
        private const string humanPlayer = "Player 1";

        public static string SwapPlayer(string currentPlayer)
        {
            if (currentPlayer == computerPlayer)
            {
                currentPlayer = humanPlayer;
            }
            else
            {
                currentPlayer = computerPlayer;
            }

            return currentPlayer;
        }

        public static void PlayGame()
        {
            bool playAnother = true;
            int total = 0;
            int final = 21;
            int roundChoice = 0;
            string currentPlayer = RandomPLayerSelect();
            int compWins = 0;
            int humanWins = 0;

            while (playAnother)
            {
                Console.WriteLine($"Now playing: {currentPlayer}");
                try
                {
                    if (currentPlayer == computerPlayer)
                    {
                       roundChoice =  CompMove(total);
                    }
                    else
                    {
                        roundChoice = int.Parse(Console.ReadLine());
                    }
                    

                    if (roundChoice != 1 && roundChoice != 2 && roundChoice != 3)
                    {
                        throw new Exception();
                    }

                    total += roundChoice;
                }
                catch (Exception)
                {
                    Console.WriteLine("Invalid choice! Choose from numbers: 1, 2, 3.");
                    continue;
                }

                Console.WriteLine(total);

                if (total == final)
                {
                    if (currentPlayer == computerPlayer)
                    {
                        compWins++;
                    }
                    if (currentPlayer == humanPlayer)
                    {
                        humanWins++;
                    }
                    Console.WriteLine($"Winner: {currentPlayer}");
                    Console.WriteLine($"Comp wins: {compWins}. Human wins: {humanWins}");
                    Console.WriteLine($"do you wan to play another round? y/n");
                    var choice = Console.ReadLine();
                    if (choice == "y")
                    {
                        total = 0;
                    }
                    else if (choice == "n")
                    {
                        break;
                    }
                    else
                    {
                        Console.WriteLine("Invalid choice! Choose from y or n");
                        continue;
                    }
                }

                else if (total > 21)
                {
                    Console.WriteLine("Not the right time to play this game :)");
                    break;
                }

                currentPlayer = SwapPlayer(currentPlayer);
            }
        }

        public static bool CheckIfCanWin(int total)
        {
            bool result = false;
            if (total == 18)
            {
                result = true;
            }
            return result;
        }

        public static int CompMove(int total)
        {
            int choice = 0;

            if (CheckIfCanWin(total))
            {
                choice = 21 - total;
            }
            else
            {
                choice = new Random().Next(1,4);
            }

            return choice;
        }

        public static string RandomPLayerSelect()
        {
            string[] players = new string[] { computerPlayer, humanPlayer };
            var random = new Random().Next(0,2);
            return players[random];
        }

        public static void Main(string[] args)
        {
            
            Console.WriteLine("Welcome to 21 game \n");
            Console.WriteLine(@"21 is a two player game. 
The game is played by choosing a number.
1, 2, or 3 to be added a total sum. \n
The game is won by the player reaches exactly 21. \n" );            
            Console.WriteLine("Choose your number: (1, 2 or 3)");

            PlayGame();
        }
    }
}