Sudoku Solver leetcode java

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     }

Reference：http://rleetcode.blogspot.com/2014/01/sudoku-solver-java.html