APCS/02 Recursion/06 NQueens/NQueens.java

//Name: B6-24   Date: 10/5/19
//
// 
//
// License Information:
//   This class is free software; you can redistribute it and/or modify
//   it under the terms of the GNU General Public License as published by
//   the Free Software Foundation.
//
//   This class is distributed in the hope that it will be useful,
//   but WITHOUT ANY WARRANTY; without even the implied warranty of
//   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
//   GNU General Public License for more details.

import edu.kzoo.grid.BoundedGrid;
import edu.kzoo.grid.Grid;
import edu.kzoo.grid.Location;
import edu.kzoo.grid.display.GridDisplay;

/**
 *  Environment-Based Applications:<br>
 *
 *    The NQueens class implements the N Queens problem. 
 *
 *  @author Your Name (based on a template provided by Alyce Brady)
 *  @version 1 September 2002
 **/

public class NQueens
{
    // Instance Variables: Encapsulated data for EACH NQueens problem
    private Grid board;
    private GridDisplay display;

  // constructor

    /** Constructs an object that solves the N Queens Problem.
     *    @param n the number of queens to be placed on an
     *              <code>n</code> x <code>n</code> board
     *    @param d an object that knows how to display an 
     *              <code>n</code> x <code>n</code> board and
     *              the queens on it
     **/
    public NQueens(int n, GridDisplay d)
    {
        board = new BoundedGrid(n, n);
        display = d;
        display.setGrid(board);
        display.showGrid();
    }

  // methods

    /** Returns the number of queens to be placed on the board. **/
    public int numQueens()
    {
        return board.numRows();   // replace this with something more useful
    }

    /** Solves (or attempts to solve) the N Queens Problem. **/
    public boolean solve()
    {
        if(placeQueen(0)) {
            display.showGrid();
            return true;
        } else {
            System.out.println("NO SOLUTION");
            display.showGrid();
            return false;   
        }
    }

    /** Attempts to place the <code>q</code>th queen on the board.
     *  (Precondition: <code>0 <= q < numQueens()</code>)
     *    @param q index of next queen to place
     **/
    private boolean placeQueen(int q)
    {
        // Queen q is placed in row q.  The only question is
        // which column she will be in.  Try them in turn.
        // Whenever we find a column that could work, put her
        // there and see if we can place the rest of the queens.

        if(q >= numQueens())
            return true;
        for(int i = 0; i < numQueens(); i++) {
            if (locationIsOK(new Location(q, i))) {
                addQueen(new Location(q, i));
                display.showGrid();
                if(placeQueen(q + 1))
                    return true;
                else
                    removeQueen(new Location(q, i));
            }
        }
        
        return false;
        
    }

    /** Determines whether a queen can be placed at the specified
     *  location.
     *    @param loc  the location to test
     **/
    private boolean locationIsOK(Location loc)
    {
         //System.out.print("I ran! ");
        // Verify that another queen can't attack this location.
        // (Only queens in previous rows have been placed.)
        int currentRow = loc.row();
        int currentCol = loc.col();
        int i, j;
        
        
        for (i = 0; i < numQueens(); i++) {
            if (board.objectAt(new Location(i, currentCol)) != null) {
                return false;
            }
        }
        
        //only have to check for above left and right because it moves down
        for (i = currentRow, j = currentCol; i >= 0 && j < numQueens(); i--, j++) {
            if (board.objectAt(new Location(i, j)) != null) {
               return false;
            }
        }
        
        for (i = currentRow, j = currentCol; i >= 0 && j >= 0; i--, j--) {
            if (board.objectAt(new Location(i, j)) != null) {
               return false;
            }
        }
        
        return true;
    }

    /** Adds a queen to the specified location.
     *    @param loc  the location where the queen should be placed
     **/
    private void addQueen(Location loc)
    {
        new Queen(board, loc);      // queens add themselves to the board
    }

    /** Removes a queen from the specified location.
     *    @param loc  the location where the queen should be removed
     **/
    private void removeQueen(Location loc)
    {
        board.remove(loc);
        // replace this with something useful.
    }

}