import java.awt.* ;
import java.awt.event.* ;
import java.applet.* ;
public class The_Eight_Queens extends Applet implements ActionListener
{
List solutionList ;
String msg="SOLUTIONS TO THE EIGHT QUEENS PROBLEM ";
String msg2="Click on a number in the List Box.";
String msg3="Then click 'View' to see the corresponding positions of Queens on the ChessBoard.";
String msg4="Or simply Double Click on a number to see the corresponding positions of Queens.";
String msg5="The Corresponding Positions of Queens are indicated by RED squares on the Board .";
Label lbl1=new Label(msg);
Label lbl2=new Label(msg2);
Label lbl3=new Label(msg3);
Label lbl4=new Label(msg4);
Label lbl5=new Label(msg5);
Button b1=new Button("View");
int equiv[]=new int[8];
int solution[] = new int[8] ;
int currentRow ; int flag=1;
public void init()
{
setLayout(new FlowLayout(FlowLayout.LEFT));
currentRow = 0 ;
solutionList=new List(8,false) ;
setBackground(Color.black);
setForeground(Color.orange);
add(lbl1);
add(lbl2);
add(lbl3);
add(lbl4); msg="";
add(solutionList); add(b1);
solutionList.addActionListener(this);
b1.addActionListener(this);
add(lbl5);
}
public void actionPerformed(ActionEvent e)
{
String msg2=solutionList.getSelectedItem();
int equivalent=Integer.parseInt(msg2);
for(int ctrn=0;ctrn<=7;ctrn++)
{
equiv[ctrn]=equivalent%10 ;
equivalent=equivalent/10;
};
repaint();
}
public void paint(Graphics g)
{
int ctr1,ctr2,row,col ;
for(ctr1=0;ctr1<=63;ctr1++)
{
row=(int)(ctr1%8);
col =(int)(ctr1 / 8 );
if((row+col)%2==0)
{
g.setColor(Color.gray);
g.fillRect(310+10*row,118+10*col,10,10);
}
else if ((row+col)%2==1)
{
g.setColor(Color.white);
g.fillRect(310+10*row,118+10*col,10,10);
};
}
if(equiv[0]!=0)
for(ctr1=0;ctr1<=7;ctr1++)
{
row=ctr1;
col = equiv[7-ctr1]-1;
g.setColor(Color.red);
g.fillRect(310+10*row,118+10*col,10,10);
};
}
public void start()
{
for(int ctr1=0;ctr1<=7;ctr1++)
placeQueens(0,ctr1);
}
public void placeQueens(int row,int col)
{
solution[row]=col ;
int ctr1,ctr2;
if(row==7)
{
displayQueens() ;
return ;
};
int flagX = 1 ;
for(ctr2=0;ctr2<=7;ctr2++)
{
flagX=1 ;
for(ctr1=0;ctr1<=row;ctr1++)
if(solution[ctr1] == ctr2 || (solution[ctr1]-ctr2)*(solution[ctr1]-ctr2)== (row+1-ctr1)*(row+1-ctr1) ) flagX=0;
if(flagX==1)
placeQueens(row+1,ctr2);
};
}
public void displayQueens()
{
msg="";
int ctr1;
for(ctr1=0;ctr1<=7;ctr1++)
msg+=(solution[ctr1]+1);
solutionList.add(msg);
}
}
