Java application to compute the number of distinct tic-tac-toe games

HowManyTicTacToeGames.java

/**
 * Compute number of distinct tic-tac-toe games.
 * This is a popular interview question.
 * With Java 11+ run as:
 * <pre>
 * java HowManyTicTacToeGames.java
 * </pre>
 * @see https://en.wikipedia.org/wiki/Tic-tac-toe
 * @author Marco Schmidt
 */
public class HowManyTicTacToeGames
{
  public static void main(String[] args)
  {
    final byte[] data = new byte[9];
    final long number = countGames(data, true);
    System.out.println(number + " games");
  }

  private static long countGames(byte[] data, boolean first)
  {
    if (isOver(data))
    {
      return 1;
    }

    long sum = 0;
    for (int i = 0; i < 9; i++)
    {
      if (data[i] == 0)
      {
        final byte[] a = new byte[9];
        System.arraycopy(data, 0, a, 0, 9);
        a[i] = first ? (byte) 1 : (byte) 2;
        sum += countGames(a, !first);
      }
    }
    if (sum == 0)
    {
      sum = 1;
    }
    return sum;
  }

  private static boolean isOver(byte[] data)
  {
    return ident(data, 0, 1, 2) || ident(data, 3, 4, 5) || ident(data, 6, 7, 8) || ident(data, 0, 3, 6)
        || ident(data, 1, 4, 7) || ident(data, 2, 5, 8) || ident(data, 0, 4, 8) || ident(data, 2, 4, 6);
  }

  private static boolean ident(byte[] data, int a, int b, int c)
  {
    return data[a] != 0 && data[a] == data[b] && data[b] == data[c];
  }
}