// return a string that can display the board nicely
// this is a convenience for a text-based interface and for debugging
  public String toString() {
    String s;
    s = "boardsize is " + boardSize + " by " + boardSize + "\n";
    if ( (winner == 'X') || (winner == 'O'))
      s = s + winner + " won!\n";
    else if (winner == 'D')
      s = s + "game is a draw\n";
    else if (playerToMove == null)
      s = s + "game not started yet\n";
    else
      s = s + "next player is " + playerToMove.getUserId() + "\n";

    for (int i = 0; i < boardSize; i++) {
      for (int j = 0; j < boardSize; j++) {
        s = s + " " + board[i][j] + " ";
        if (j < boardSize - 1)
          s = s + "|";
      }
      s = s + "\n";
      if (i < boardSize - 1) {
        for (int j = 0; j < boardSize; j++) {
          s = s + "---";
          if (j < boardSize - 1)
            s = s + "|";
        }
        s = s + "\n";
      }
    }
    return s;
  }