八皇后解法（回溯法）

Posted 2020-08-09

package com.company;

/**
 * Created by Administrator on 2016/9/15.
 */
public class EigthQueue {
    private static int N = 8;
    private int count = 0; // 总方案数
    private int[] flag = {-1, -1, -1, -1, -1, -1, -1, -1};
    //回溯法递归实现八皇后问题
    //输出棋盘
    private void printChessboard(){
        int [][]chessboard = new int[N][N];
        for ( int i=0; i<N; i++) {
            chessboard[i][flag[i]] = 1;
        }
        for ( int i=0; i<N; i++) {
            for ( int j=0; j<N; j++) {
                System.out.print(String.format("%d ", chessboard[i][j]));
            }
            System.out.println();
        }
    }
    private boolean checkQueen(int p){
        boolean flag_t = true;
        int pv = flag[p];
        if ( p==0) {
            return flag_t;
        }
        p--;
        for ( int i=1; p>=0; p--, i++) {
            if ( flag[p]==pv || flag[p]==pv-i || flag[p]==pv+i) {
                flag_t = false;
            }
        }
        return flag_t;
    }
    public void eightQueen(int p){
        for ( int i=0; i<N; i++) {
            flag[p] = i;
            if ( checkQueen( p)==true && p==7) {
                count++;
                System.out.println("第" + count + "种:");
                printChessboard( );
            }
            else if ( checkQueen( p ) == true){
                eightQueen(p+1);
            }
        }
    }

    public static void main(String[]args){
        // 记录开始时间
        long a=System.currentTimeMillis();
        EigthQueue eigthQueue = new EigthQueue();
        eigthQueue.eightQueen(0);
        // 记录结束时间
        System.out.println("\r<br>执行耗时 : "+(System.currentTimeMillis()-a)/1000f+" 秒 ");
    }
}