main.java

一个比较完整的八皇后Java求解问题

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package eightqueens;

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

class Queen {

    private int row;
    private int column;
    private Queen neighbor;

    Queen(int c, Queen n) {
        row = 1;
        column = c;
        neighbor = n;
    }

    /*
    public boolean findSolution() {
    while (neighbor != null && neighbor.canAttack(row, column)) {
    // If attacking exists, try to advance  
    if (!advance()) {
    // If cannot advance
    return false;
    }
    // If advance succeeded, check attacking again
    }
    return true;
    }
    public boolean advance() {
    if (row < Main.QUEEN_COUNT) {
    row++;
    // try to find solution again
    return findSolution();
    }
    // Back trace
    if (neighbor != null) {
    if (!neighbor.advance()) {
    return false;
    }
    if (!neighbor.findSolution()) {
    return false;
    }
    // Restart from row 1
    row = 1;
    return findSolution();
    } else {
    // Back to the first row
    return false;
    }
    }
     */
    public boolean findSolution() {
        while (neighbor != null && neighbor.canAttack(this.row, this.column)) {
            // If attacking exists, try to advance  
            if (!advance()) {
                // If cannot advance
                return false;
            }
        // If advance succeeded, check attacking again
        }

        // No attacking now!
        return true;
    }

    public boolean advance() {
        if (this.row < Main.QUEEN_COUNT) {
            this.row++;
            return true;
        }

        if (neighbor == null) {
            // This is the first queen.
            return false;
        }

        // Back trace
        if (!neighbor.advance()) {
            return false;
        }
        
        if (!neighbor.findSolution()) {
            return false;
        }

        // Restart from row 1
        this.row = 1;
        return true;
    }

    private boolean canAttack(int testRow, int testColumn) {
        int columnDifference = testColumn - column;

        if (row == testRow ||
                row + columnDifference == testRow ||
                row - columnDifference == testRow) {
            return true;
        }

        if (neighbor != null) {
            return neighbor.canAttack(testRow, testColumn);
        }

        return false;
    }

    void paint(Graphics g) {
        if (neighbor != null) {
            neighbor.paint(g);
        }

        int x = (row - 1) * 50 + 10;
        int y = (column - 1) * 50 + 40;

        g.drawLine(x + 5, y + 45, x + 45, y + 45);
        g.drawLine(x + 5, y + 45, x + 5, y + 5);
        g.drawLine(x + 45, y + 45, x + 45, y + 5);
        g.drawLine(x + 5, y + 35, x + 45, y + 35);
        g.drawLine(x + 5, y + 5, x + 15, y + 20);
        g.drawLine(x + 15, y + 20, x + 25, y + 5);
        g.drawLine(x + 25, y + 5, x + 35, y + 20);
        g.drawLine(x + 35, y + 20, x + 45, y + 5);
        g.drawLine(x + 20, y + 20, x + 10, y + 10);

    }
}

/**
 *
 * @author qingxl
 */
public class Main extends JFrame {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        Main world = new Main();

        world.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        world.setVisible(true);
    }

    public Main() {
        setTitle(QUEEN_COUNT + " Queens");
        setSize(600, 500);

        boolean found = false;

        for (int i = 1; i <= QUEEN_COUNT; i++) {
            lastQueen = new Queen(i, lastQueen);
            found = lastQueen.findSolution();
            if (!found) {
                break;
            }
        }

        displayResult(found);

        addMouseListener(new MouseKeeper());
    }

    public void paint(Graphics g) {
        super.paint(g);

        for (int i = 0; i <= QUEEN_COUNT; i++) {
            g.drawLine(50 * i + 10, 40, 50 * i + 10, 40 + QUEEN_COUNT * 50);
            g.drawLine(10, 50 * i + 40, 10 + QUEEN_COUNT * 50, 50 * i + 40);

        }

        g.drawString("Click mouse for the next solution", 20, 70 + QUEEN_COUNT * 50);
        lastQueen.paint(g);
    }

    private void displayResult(boolean found) {
        if (found) {
            // Now, find a solution!
            JOptionPane.showMessageDialog(null, "Found a solution!");
            System.out.println("Found a solution!");
        } else {
            JOptionPane.showMessageDialog(null, "No solution!");
            System.out.println("No solution!");
        }
    }

    private class MouseKeeper extends MouseAdapter {

        public void mousePressed(MouseEvent e) {
            boolean found = lastQueen.advance() && lastQueen.findSolution();

            repaint();

            displayResult(found);
        }
    }
    private Queen lastQueen = null;
    public static final int QUEEN_COUNT = 4;
}