I am trying to implement the Sudoku puzzle solver using Java but below code is failing on second case (it should return false). From the output being printed, it looks like the code exits even before the entire grid is solved. What am I missing?
Can someone help check this quickly and hint in case you see a problem.
Question: Write the function sudokuSolve that checks whether a given sudoku board (i.e. 9x9 sudoku puzzle) is solvable. If so, the function will returns true. Otherwise (i.e. there is no valid solution to the given sudoku board), returns false.
code:
public class Sudoku {
static boolean sudokuSolve(char[][] board) {
int row = -1, col = -1;
List<Integer> candidates = null; // store smallest possible candidates
for (int r = 0; r < 9; r++) {
for (int c = 0; c < 9; c++) {
if (board[r][c] == '.') {
List<Integer> newCandidates = getCandidates(board, r, c);
if (candidates == null || newCandidates.size() < candidates.size()) {
candidates = newCandidates;
row = r;
col = c;
}
}
}
}
if (candidates == null || candidates.isEmpty())
return true;
for (Integer val : candidates) {
board[row][col] = (char) (val.intValue() + '0');
if (sudokuSolve(board))
return true;
else
board[row][col] = '.';
}
printGrid(board);
return false;
}
/**
* For each empty cell, consider 'newCandidates', the set of possible candidate
* values that can be placed into that cell.
*
*/
public static List<Integer> getCandidates(char[][] board, int x, int y) {
// filled row numbers
boolean[] mask = new boolean[10];
for (int row = 0; row < 9; row++) {
char ch = board[row][y];
if (ch != '.')
mask[ch - '0'] = true;
}
// filled col numbers
for (int col = 0; col < 9; col++) {
char ch = board[x][col];
if (ch != '.')
mask[ch - '0'] = true;
}
// filled subgrid numbers
// starts at row - row%3, col - col%3
int subBoardRow = x - (x % 3);
int subBoardCol = y - (y % 3);
for (int row = 0; row < 3; row++) {
for (int col = 0; col < 3; col++) {
char ch = board[subBoardRow + row][subBoardCol + col];
if (ch != '.')
mask[ch - '0'] = true;
}
}
List<Integer> candidates = new ArrayList<Integer>();
for (int i = 1; i <= 9; i++) {
if (!mask[i])
candidates.add(i);
}
return candidates;
}
public static void main(String[] args) {
char[][] board = new char[][] {
{'.','.','.','7','.','.','3','.','1'},
{'3','.','.','9','.','.','.','.','.'},
{'.','4','.','3','1','.','2','.','.'},
{'.','6','.','4','.','.','5','.','.'},
{'.','.','.','.','.','.','.','.','.'},
{'.','.','1','.','.','8','.','4','.'},
{'.','.','6','.','2','1','.','5','.'},
{'.','.','.','.','.','9','.','.','8'},
{'8','.','5','.','.','4','.','.','.'}};
System.out.println(sudokuSolve(board));
printGrid(board);
System.out.println("***************************************");
char[][] board2 = new char[][] {
{'.','8','9','.','4','.','6','.','5'},
{'.','7','.','.','.','8','.','4','1'},
{'5','6','.','9','.','.','.','.','8'},
{'.','.','.','7','.','5','.','9','.'},
{'.','9','.','4','.','1','.','5','.'},
{'.','3','.','9','.','6','.','1','.'},
{'8','.','.','.','.','.','.','.','7'},
{'.','2','.','8','.','.','.','6','.'},
{'.','.','6','.','7','.','.','8','.'}};
System.out.println(sudokuSolve(board2));
printGrid(board2);
}
public static void printGrid(char[][] grid) {
for (int r = 0; r < grid.length; r++) {
for (int c = 0; c < grid[0].length; c++) {
System.out.print(grid[r][c] + " ");
}
System.out.print("\n");
}
}
}
Output:
true
6 5 8 7 4 2 3 9 1
3 1 2 9 8 6 4 7 5
7 4 9 3 1 5 2 8 6
2 6 3 4 9 7 5 . .
. . . 1 . . . . .
. . 1 2 . 8 . 4 .
. . 6 8 2 1 . 5 .
. . . 5 . 9 . . 8
8 . 5 6 . 4 . . .
***************************************
true 1 8 9 2 4 3 6 7 5 2 7 3 5 6 8 9 4 1 5 6 4 9 1 7 2 3 8 4 1 2 7 3 5 8 9 6 6 9 8 4 2 1 7 5 3 7 3 5 9 8 6 4 1 2 8 4 1 . 5 9 . 2 7 . 2 7 8 . . . 6 . . . 6 . 7 . . 8 .
