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();
        }
    }
}

Java: import java.util.Random;
import java.util.Scanner;

public class TwentyOneGame {

    public static void main(String[] args) {
        new TwentyOneGame().run(true, 21, new int[] {1, 2, 3});
    }
    
    public void run(boolean computerPlay, int max, int[] valid) {
        String comma = "";
        for ( int i = 0 ; i < valid.length ; i++ ) {
            comma += valid[i];
            if ( i < valid.length - 2 && valid.length >= 3 ) {
                comma += ", ";
            }
            if ( i == valid.length - 2 ) {
                comma += " or ";
            }
        }
        System.out.printf("The %d game.%nEach player chooses to add %s to a running total.%n" + 
                "The player whose turn it is when the total reaches %d will win the game.%n" + 
                "Winner of the game starts the next game.  Enter q to quit.%n%n", max, comma, max);
        int cGames = 0;
        int hGames = 0;
        boolean anotherGame = true;
        try (Scanner scanner = new Scanner(System.in);) {
            while ( anotherGame ) {
                Random r = new Random();
                int round = 0;
                int total = 0;
                System.out.printf("Start game %d%n", hGames + cGames + 1);
                DONE:
                    while ( true ) {
                        round++;
                        System.out.printf("ROUND %d:%n%n", round);
                        for ( int play = 0 ; play < 2 ; play++ ) {
                            if ( computerPlay ) {
                                int guess = 0;
                                
                                for ( int test : valid ) {
                                    if ( total + test == max ) {
                                        guess = test;
                                        break;
                                    }
                                }
                                
                                if ( guess == 0 ) {
                                    for ( int test : valid ) {
                                        if ( total + test >= max ) {
                                            guess = test;
                                            break;
                                        }
                                    }
                                }
                                if ( guess == 0 ) {
                                    guess = valid[r.nextInt(valid.length)];
                                }
                                total += guess;
                                System.out.printf("The computer chooses %d%n", guess);
                                System.out.printf("Running total is now %d%n%n", total);
                                if ( total >= max ) {
                                    break DONE;
                                }
                            }
                            else {
                                while ( true ) {
                                    System.out.printf("Your choice among %s: ", comma);
                                    String line = scanner.nextLine();
                                    if ( line.matches("^[qQ].*") ) {
                                        System.out.printf("Computer wins %d game%s, human wins %d game%s.  One game incomplete.%nQuitting.%n", cGames, cGames == 1 ? "" : "s", hGames, hGames == 1 ? "" : "s");
                                        return;
                                    }
                                    try {
                                        int input = Integer.parseInt(line);
                                        boolean inputOk = false;
                                        for ( int test : valid ) {
                                            if ( input == test ) {
                                                inputOk = true;
                                                break;
                                            }
                                        }
                                        if ( inputOk ) {
                                            total += input;
                                            System.out.printf("Running total is now %d%n%n", total);
                                            if ( total >= max ) {
                                                break DONE;
                                            }
                                            break;
                                        }
                                        else {
                                            System.out.printf("Invalid input - must be a number among %s.  Try again.%n", comma);
                                        }
                                    }
                                    catch (NumberFormatException e) {
                                        System.out.printf("Invalid input - must be a number among %s.  Try again.%n", comma);
                                    }
                                }
                            }
                            computerPlay = !computerPlay;
                        }
                    }
                String win;
                if ( computerPlay ) {
                    win = "Computer wins!!";
                    cGames++;
                }
                else {
                    win = "You win and probably had help from another computer!!";
                    hGames++;
                }
                System.out.printf("%s%n", win);
                System.out.printf("Computer wins %d game%s, human wins %d game%s%n%n", cGames, cGames == 1 ? "" : "s", hGames, hGames == 1 ? "" : "s");
                while ( true ) {
                    System.out.printf("Another game (y/n)? ");
                    String line = scanner.nextLine();
                    if ( line.matches("^[yY]$") ) {
                        
                        System.out.printf("%n");
                        break;
                    }
                    else if ( line.matches("^[nN]$") ) {
                        anotherGame = false;
                        System.out.printf("Quitting.%n");
                        break;
                    }
                    else {
                        System.out.printf("Invalid input - must be a y or n.  Try again.%n");
                    }
                }
            }
        }
    }

}