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