回溯法一定要记得恢复原值，因为前后值都有关联
有一种牛逼八皇后不用这样
if(solve(board))
return true; //If it's the solution return true
else
board[i][j] = '.'; //Otherwise go back
}
是因为它另拿了一个数组存结果，没有利用原来的棋盘
public class Solution {
public void solveSudoku(char[][] board) {
if(board == null || board.length == 0)
return;
solve(board);
}
public boolean solve(char[][] board){
for(int i = 0; i < board.length; i++){
for(int j = 0; j < board[0].length; j++){
if(board[i][j] == '.'){
for(char c = '1'; c <= '9'; c++){//trial. Try 1 through 9
if(isValid(board, i, j, c)){
board[i][j] = c; //Put c for this cell
if(solve(board))
return true; //If it's the solution return true
else
board[i][j] = '.'; //Otherwise go back
}
}
return false;
}
}
}
return true;
}
private boolean isValid(char[][] board, int row, int col, char c){
for(int i = 0; i < 9; i++) {
if(board[i][col] != '.' && board[i][col] == c) return false; //check row
if(board[row][i] != '.' && board[row][i] == c) return false; //check column
if(board[3 * (row / 3) + i / 3][ 3 * (col / 3) + i % 3] != '.' &&
board[3 * (row / 3) + i / 3][3 * (col / 3) + i % 3] == c) return false; //check 3*3 block
}
return true;
}
}
class Solution {
public void solveSudoku(char[][] board) {
boolean res=backtrack(board);
}
private boolean backtrack(char[][] board){
for(int i=0;i<9;i++){
for(int j=0;j<9;j++){
if(board[i][j]=='.'){
for(int k=1;k<=9;k++){
board[i][j]=(char)(k+48);
if(isValidSudoku(board)){
if(backtrack(board)) return true;
board[i][j]='.';
}
else{
board[i][j]='.';
}
}
return false;
}
}
}
return true;
}
public boolean isValidSudoku(char[][] board) {
for(int i=0;i<9;i+=3){
for(int j=0;j<9;j+=3){
if(!CheckSquare(board,i,j)){
return false;
}
}
}
for(int i=0;i<9;i++){
if(!CheckRow(board,i)){
return false;
}
}
for(int j=0;j<9;j++){
if(!CheckColumn(board,j)){
return false;
}
}
return true;
}
private boolean CheckRow(char[][] board,int i){
Set<Character> set=new HashSet<>();
for(int j=0;j<9;j++){
if(board[i][j]!='.'){
if(!set.contains(board[i][j])){
set.add(board[i][j]);
}
else{
return false;
}
}
}
return true;
}
private boolean CheckColumn(char[][] board,int j){
Set<Character> set=new HashSet<>();
for(int i=0;i<9;i++){
if(board[i][j]!='.'){
if(!set.contains(board[i][j])){
set.add(board[i][j]);
}
else{
return false;
}
}
}
return true;
}
private boolean CheckSquare(char[][] board,int i,int j){
Set<Character> set=new HashSet<>();
int p=i,q=j;
for(p=i;p<i+3;p++){
for(q=j;q<j+3;q++){
if(board[p][q]!='.'){
if(!set.contains(board[p][q])){
set.add(board[p][q]);
}
else{
return false;
}
}
}
}
return true;
}
}