Sudoku Solver

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...

 

Thoughts:

1.write a backtracking recursive method to attemp to fill out every possible number into the empty board, if the number is not valid, return false and keep recursively filling out other numbers till the first possibly results come up.

2. write a generic function to validdate the input position of number.

 

Code:

private static final int[] mask = new int[]{0,1,2,4,8,16,32,64,128,256};
public void solveSudoku(char[][] board) {
List<byte[]> avPoints = new ArrayList<byte[]>();
for (byte i=0;i<board.length;i++){
for (byte j=0;j<board.length;j++){
if (board[i][j] == '.'){
avPoints.add(new byte[]{i,j});
}
}
}
solve(board, avPoints);
}
private boolean solve(char[][] board, List<byte[]> points){  // recursive function
if (points.isEmpty()){
return true;
}
byte[] point = points.remove(points.size()-1); //remove at the end for speed
int notAvailable = getNotAvailable(point, board);
for (int i=1;i<=9;i++){
if ((notAvailable&mask[i]) == 0){//number is available
board[point[0]][point[1]] = (char)('0' + i);
if (solve(board, points)){
return true; //unwind
}
board[point[0]][point[1]] = '.'; //backtrack
}
}
points.add(point); //backtrack
return false;
}
private int getNotAvailable(byte[] point, char[][] board){    // Validation fucntion
int result = 0;
int x1 = 3*(point[0]/3);
int y1 = 3*(point[1]/3);
for (int m=0;m<9;m++){
if (board[point[0]][m]!='.'){
result|=mask[board[point[0]][m]-'0'];
}
if (board[m][point[1]]!='.'){
result|=mask[board[m][point[1]]-'0'];
}
int xBox=x1 + m/3;
int yBox=y1 + m%3;
if(board[xBox][yBox]!='.'){
result|=mask[board[xBox][yBox]-'0'];
}
}
return result;
}