public class KnightsTour{

    private int[][] board;
    private final static int BOARD_SIZE = 8;
    private int num = 0; // the current knight to be placed

    public KnightsTour(){
	board = new int[BOARD_SIZE][BOARD_SIZE];
    }

    private boolean free(int row, int column){
	if (row<0 || row>=BOARD_SIZE || column<0 || column>=BOARD_SIZE)
	    return false;
	else 
	    return board[row][column] == 0;
    }

    private boolean tour(int row, int column){
	boolean done = false;

	if (free(row,column)){
	    num ++;
	    board[row][column] = num;
	    //	    System.out.println(this);

	    if (num == BOARD_SIZE * BOARD_SIZE)   // tour finished
		done = true;
	    else{
		done = tour (row-2, column-1);
		if (!done)
		    done = tour (row-2, column+1);
		if (!done)
		    done = tour (row-1, column-2);
		if (!done)
		    done = tour (row-1, column+2);
		if (!done)
		    done = tour (row+1, column-2);
		if (!done)
		    done = tour (row+1, column+2);
		if (!done)
		    done = tour (row+2, column-1);
		if (!done)
		    done = tour (row+2, column+1);
	    }
	    if (!done){ // we must backtrack
		board[row][column] = 0;  // erase knight
		num --;  // num of knights placed decremented
		//		System.out.println(this);
	    } 
	}
	return done;
    }

    public String toString(){
	String ret = "";
	for (int r=0; r<BOARD_SIZE; r++){
	    for (int c=0; c<BOARD_SIZE; c++)
		if (board[r][c]>9)
		    ret += " " + board[r][c];
		else 
		    ret += "  " + board[r][c];
	    ret += "\n";
	}
	return ret;
    } 

    public static void main (String[] args){
	KnightsTour K = new KnightsTour();
	if (K.tour(0,0))
	    System.out.println(K);
	else 
	    System.out.println("No knight's tour possible on a " +
			       BOARD_SIZE + " x " + BOARD_SIZE + " board.");  
    }

}