从经典的八皇后问题看回溯

 

package backtracking;

/**

 *运用递归做的"八皇后"经典问题,掌握回溯思想

 * @author LeLe

 *

 */

public class EightQueen {

 

    //竖线

    boolean[] line = new boolean[4];

    //左对角线

    boolean[] leftDiagonal = new boolean[7];

    //右对角线

    boolean[] rightDiagonal = new boolean[7];

   //y坐标存储

    int[] lineS = new int[4];

   

    public EightQueen(){

        line[0] = line[1] = line[2] = line[3] = true;

        for (int i = 0; i < leftDiagonal.length&&i < rightDiagonal.length; i++) {

            leftDiagonal[i] = true;

            rightDiagonal[i] = true;

        }

    }

    public void putQueen(int row){

        for (int i = 0; i < line.length; i++) {

            if(line[i]==true&&leftDiagonal[row+i]==true&&rightDiagonal[row-i+3]==true){

                //操作成功

                line[i] = false;

                leftDiagonal[row+i] = false;

                rightDiagonal[row-i+3] = false;

                lineS[row] = i;

                if(row<line.length-1){

                    this.putQueen(row+1);

                }else if(row == line.length-1){

                    System.out.println("$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$");

                    for (int j = 0; j < line.length; j++) {

                        System.out.println("皇后坐标为"+j+" "+lineS[j]);

                    }

                    System.out.println("$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$");

                }

                line[i] = true;

                leftDiagonal[row+i] = true;

                rightDiagonal[row-i+3] = true;

            }

        }

    }

   

    public static void main(String[] args){

        EightQueen queen = new EightQueen();

        long a = System.currentTimeMillis();

        queen.putQueen(0);

        long b = System.currentTimeMillis();

        System.out.println("执行时间为 "+(b-a)+"毫秒");

    }

}