题目：
Write a program to solve a Sudoku puzzle by filling the empty cells.
Empty cells are indicated by the character '.'.
You may assume that there will be only one unique solution.
A sudoku puzzle...
...and its solution numbers marked in red.
题解：
第一反应就是N皇后问题。就是一点点尝试着填数，不行的话就回溯，直到都填满就返回。
如果对一个格子尝试从0~9都不行，那么说明整个sudoku无解，返回false就好。
对整个棋盘所有'.'都填完了，那么就可以返回true了。
1 public void solveSudoku(char[][] board) {
2 if (board==null||board.length==0)
3 return;
4 helper(board);
5 }
6
7 private boolean helper(char[][] board){
8 for(int i=0; i<board.length; i++){
9 for (int j=0; j<board[0].length; j++){
10 if (board[i][j]=='.'){
11 for (char num='1'; num<='9'; num++){//尝试
12 if(isValid(board, i, j, num)){
13 board[i][j]=num;
14
15 if (helper(board))
16 return true;
17 else
18 board[i][j]='.';//回退
19 }
20 }
21 return false;
22 }
23 }
24 }
25
26 return true;
27 }
28
29 private boolean isValid(char[][] board, int i, int j, char c){
30 // check column
31 for (int row=0; row<9; row++)
32 if (board[row][j] == c)
33 return false;
34
35 // check row
36 for (int col=0; col<9; col++)
37 if (board[i][col]==c)
38 return false;
39
40 // check block
41 for(int row=i/3*3; row<i/3*3+3; row++)
42 for (int col=j/3*3; col<j/3*3+3; col++)
43 if (board[row][col]==c)
44 return false;
45
46 return true;
47 }
2 if (board==null||board.length==0)
3 return;
4 helper(board);
5 }
6
7 private boolean helper(char[][] board){
8 for(int i=0; i<board.length; i++){
9 for (int j=0; j<board[0].length; j++){
10 if (board[i][j]=='.'){
11 for (char num='1'; num<='9'; num++){//尝试
12 if(isValid(board, i, j, num)){
13 board[i][j]=num;
14
15 if (helper(board))
16 return true;
17 else
18 board[i][j]='.';//回退
19 }
20 }
21 return false;
22 }
23 }
24 }
25
26 return true;
27 }
28
29 private boolean isValid(char[][] board, int i, int j, char c){
30 // check column
31 for (int row=0; row<9; row++)
32 if (board[row][j] == c)
33 return false;
34
35 // check row
36 for (int col=0; col<9; col++)
37 if (board[i][col]==c)
38 return false;
39
40 // check block
41 for(int row=i/3*3; row<i/3*3+3; row++)
42 for (int col=j/3*3; col<j/3*3+3; col++)
43 if (board[row][col]==c)
44 return false;
45
46 return true;
47 }
Reference：http://rleetcode.blogspot.com/2014/01/sudoku-solver-java.html