问题描述

八皇后问题__java：求问这里出现的异常问题

关于8皇后问题，这里为了简便就是4个皇后在排列，问题出在我每次把程序里的Net数组加到net set里面以后，就发生了改变，这就导致后续的重复方式被加入net set
(HashSet类型)里面，求问为什么会出现加入之后数组会发生改变？
下面是代码：

package chessloc;

import java.util.ArrayList;
import java.util.HashSet;
//net_set???????

class point {

int x;
int y;

point(int x, int y) {
    this.x = x;
    this.y = y;
}

}

public class Chessloc {

static int n = 4;
//net_set is for storing all of the possible way.
static HashSet<int[][]> net_set = new HashSet();
//remove is for recording the impossible place after one queen place.
static ArrayList<point> remove[][] = new ArrayList[n][n];
//Net is for recording the possible way for placing queen, each time finish it will be added into net_set.
static int[][] Net = new int[n][n];

public static void main(String[] args) {
    for (int p = 0; p < n; p++) {
        for (int m = 0; m < n; m++) {
            //here is for initialize reomve[][].
            for (int p_t = 0; p_t < n; p_t++) {
                for (int m_t = 0; m_t < n; m_t++) {
                    remove[p_t][m_t] = new ArrayList();
                }
            }
            Net = re();
            backtrace(p, m);
        }
    }
    System.out.println("final:");
    print();
    System.out.print(net_set.size());
}

static void backtrace(int index_x, int index_y) {
    Net[index_x][index_y] = 6;
    for (int i = 0; i < n; i++) {
        for (int m = 0; m < n; m++) {
            if (Net[i][m] == 0 && (Math.abs(index_x - i) == Math.abs(index_y - m) || i == index_x || m == index_y)) {
                point p = new point(i, m);
                remove[index_x][index_y].add(p);
                Net[i][m] = 1;
            }
        }
    }
    for (int x = 0; x < n; x++) {
        for (int y = 0; y < n; y++) {
            //a second point.
            if (Net[x][y] == 0) {
                backtrace(x, y);
                Net[x][y] = 0;
                remove(remove[x][y]);
                remove[x][y].clear();
            }
        }
    }
    if (isDone()) {
        System.out.println("correct case_Net:");
        Print();
        net_set.add(Net);
        System.out.println("After add in__Net:");
        Print();
        System.out.println("add inside_net_set:");
        print();
    }
}

//judge whether get the correct way.
public static boolean isDone() {
    int x = 0;
    for (int p = 0; p < n; p++) {
        for (int m = 0; m < n; m++) {
            x = (Net[p][m] == 6) ? x + 1 : x;
            if (Net[p][m] == 0) {
                return false;
            }
        }
    }
    return (x == n);
}

//here is for making the impossible place(under a way to place queen) become possible.
public static void remove(ArrayList<point> x) {
    for (point temp : x) {
        if (Net[temp.x][temp.y] == 1) {
            Net[temp.x][temp.y] = 0;
        }
    }
}

public static int[][] re() {
    int[][] N = new int[n][n];
    for (int p = 0; p < n; p++) {
        for (int m = 0; m < n; m++) {
            N[p][m] = 0;
        }
    }
    return N;
}

//here is for printing Net array.
public static void Print() {
    for (int p = 0; p < n; p++) {
        for (int m = 0; m < n; m++) {
            System.out.print(Net[p][m] + " ");
        }
        System.out.println("");
    }
}

//here is for printing net_set.
public static void print() {
    for (int[][] temp : net_set) {
        for (int p = 0; p < n; p++) {
            for (int m = 0; m < n; m++) {
                System.out.print(temp[p][m] + " ");
            }
            System.out.println("");
        }
        System.out.println();
    }
}

}
下面运行截图：

出现第一种case的时候运行运行正常
但是出现第二种case后，加入net set之后一种case就发生了改变。。。。。。、
求问如何修改？
对了，这个方法不是很好，效率很低，但是只是想修复它。。
谢谢了。

解决方案

因为你的hashset里的net指的是同一个位置的数组，这样就行了，加入的一个新的数组实例

 if (isDone()) {
        System.out.println("correct case_Net:");
        Print();
        int[][] Net2=new int[4][4];
        for(int i=0;i<4;i++){
            for(int j=0;j<4;j++){
                Net2[i][j]=Net[i][j];
            }
        }
        net_set.add(Net2);
        System.out.println("After add in__Net:");
        Print();
        System.out.println("add inside_net_set:");
        print();
    }