public bool IsEqualTo(IVertex state)
{
if (state == null) return false;
if (!(state is TicTacToe)) return false;
TicTacToe tictactoe = (TicTacToe)state;
for (int i = 0; i < _board.Length; i++)
if (_board[i] != tictactoe._board[i])
return false;
return true;
}