import java.util.Calendar; import java.util.Date; public class Matrix {  private int matrix[][];  private long timeAfter=0;  private long timeBefore =0;  public Matrix(int m[][]) {   matrix = new int[9][9];   for (int i=0; i<9 ; i++)    for(int j= 0; j<9; j++)           matrix[i][j]=m[i][j];   this.timeBefore = Calendar.getInstance().getTimeInMillis();  }  public void backTrack(int i, int j)  {   //回收系统内存资源   System.gc();   if( i==8 && j>=9 )   {    this.timeAfter = Calendar.getInstance().getTimeInMillis();    //成功输出矩阵    this.showMatrix();    return;   }   if(j == 9) {j = 0; i++;}   if(matrix[i][j] == 0)   {    //数字为零       for(int k=1; k<=9; k++)       {        if(bound(i,j,k))        {         matrix[i][j] = k ;         //符合条件，查找下一个方格         backTrack(i,j+1);         matrix[i][j] = 0 ;        }       }   }else   {       //数字不为零，直接查找下一个       backTrack(i, j+1);   }  }  /**   * 判断要填入的数字和同行同列以及同一九宫格内数字是否重复   */  private boolean bound(int i, int j, int k) {   int m = i/3;   int n = j/3;   for(int p = 0; p<9; p++)   {    if(k == matrix[i][p])    {     return false;    }    if(k == matrix[p][j])    {     return false;    }    if(k == matrix[3*m+p/3][3*n+p%3])    {     return false;    }   }   return true;  }  /**   * 打印解题时间   * @return   */  public long printTime()  {   return this.timeAfter-this.timeBefore;  }  /**   * 打印矩阵   */  public void showMatrix()  {   for(int i=0; i<9; i++)   {    for(int j=0; j<9; j++)    {     System.out.print(matrix[i][j]+" ");    }    System.out.println ();   }   System.out.println ();      System.out.println("解题时间： "+printTime()+"毫秒");   System.out.println ();  }  public static void main(String[] args) {      int matrix[][] = {        {3,0,6,0,5,7,0,0,0},        {7,9,0,0,2,4,0,0,0},        {0,5,0,6,0,0,9,7,4},        {8,0,1,0,0,9,0,0,0},        {0,2,0,3,0,8,0,0,7},        {4,0,0,0,6,0,5,0,0},        {0,0,4,0,3,6,0,5,0},        {2,0,3,7,0,5,0,0,1},        {0,0,7,4,1,0,6,0,0}};      int ma1[][]={      {0,3,0,0,0,5,0,6,0},      {0,1,0,0,0,3,0,8,0},      {0,4,0,0,0,0,0,0,7},      {0,0,7,0,2,4,0,0,0},      {5,0,0,0,9,0,0,0,0},      {0,8,0,3,0,0,5,0,0},      {0,0,0,8,0,0,0,0,0},      {0,0,9,0,0,0,0,7,3},      {0,5,0,9,0,0,0,0,2}};      int ma2[][]={      {0,0,0,0,8,4,0,0,0},//8      {0,0,0,2,0,3,0,8,0},      {8,3,0,9,0,0,0,5,0},      {0,5,3,0,9,0,7,0,0},      {0,0,0,6,3,7,0,4,5},//7      {0,7,0,5,0,0,0,0,0},      {0,0,6,8,0,0,0,0,0},      {3,0,0,0,2,9,0,0,0},      {2,0,9,3,0,0,0,0,1}};//3      // 号称世界上最难数独      int[][] sudoku = {      { 8, 0, 0, 0, 0, 0, 0, 0, 0 },      { 0, 0, 3, 6, 0, 0, 0, 0, 0 },      { 0, 7, 0, 0, 9, 0, 2, 0, 0 },      { 0, 5, 0, 0, 0, 7, 0, 0, 0 },      { 0, 0, 0, 0, 4, 5, 7, 0, 0 },      { 0, 0, 0, 1, 0, 6, 0, 3, 0 },      { 0, 0, 1, 0, 0, 0, 0, 6, 8 },       { 0, 0, 8, 5, 0, 0, 0, 1, 0 },      { 0, 9, 0, 0, 0, 0, 4, 0, 0 }};      Matrix m = new Matrix(sudoku);      m.backTrack(0, 0);  } }