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++){

                 if (board[i][j]=='.'){

                     for (char num='1'; num<='9'; num++){//尝试

                         if(isValid(board, i, j, num)){

                             board[i][j]=num;

                             

                             if (helper(board))

                                 return true;

                             else

                                 board[i][j]='.';//回退

                         }

                     }

                     return false;

                 }

             }

         }

         

         return true;

     }

     

     private boolean isValid(char[][] board, int i, int j, char c){

         // check column

         for (int row=0; row<9; row++)

             if (board[row][j] == c)

                 return false;

         

        // check row

         for (int col=0; col<9; col++)

             if (board[i][col]==c)

                 return false;

       

         // check block

         for(int row=i/3*3; row<i/3*3+3; row++)

             for (int col=j/3*3; col<j/3*3+3; col++)

                 if (board[row][col]==c)

                     return false;

                     

         return true;

     }

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