Crie um Sudoku Solver em Java

private static int BOARD_SIZE = 9;
private static int SUBSECTION_SIZE = 3;
private static int NO_VALUE = 0;
private static int CONSTRAINTS = 4;
private static int MIN_VALUE = 1;
private static int MAX_VALUE = 9;
private static int COVER_START_INDEX = 1;

private int getIndex(int row, int column, int num) {
    return (row - 1)* BOARD_SIZE *BOARD_SIZE
      + (column - 1)* BOARD_SIZE + (num - 1);
}

private boolean[][] createExactCoverBoard() {
    boolean[][] coverBoard = new boolean
      [BOARD_SIZE *BOARD_SIZE* MAX_VALUE]
      [BOARD_SIZE *BOARD_SIZE* CONSTRAINTS];

    int hBase = 0;
    hBase = checkCellConstraint(coverBoard, hBase);
    hBase = checkRowConstraint(coverBoard, hBase);
    hBase = checkColumnConstraint(coverBoard, hBase);
    checkSubsectionConstraint(coverBoard, hBase);

    return coverBoard;
}

private int checkSubsectionConstraint(boolean[][] coverBoard, int hBase) {
    for (int row = COVER_START_INDEX; row <= BOARD_SIZE; row += SUBSECTION_SIZE) {
        for (int column = COVER_START_INDEX; column <= BOARD_SIZE; column += SUBSECTION_SIZE) {
            for (int n = COVER_START_INDEX; n <= BOARD_SIZE; n++, hBase++) {
                for (int rowDelta = 0; rowDelta < SUBSECTION_SIZE; rowDelta++) {
                    for (int columnDelta = 0; columnDelta < SUBSECTION_SIZE; columnDelta++) {
                        int index = getIndex(row + rowDelta, column + columnDelta, n);
                        coverBoard[index][hBase] = true;
                    }
                }
            }
        }
    }
    return hBase;
}

private int checkColumnConstraint(boolean[][] coverBoard, int hBase) {
    for (int column = COVER_START_INDEX; column <= BOARD_SIZE; c++) {
        for (int n = COVER_START_INDEX; n <= BOARD_SIZE; n++, hBase++) {
            for (int row = COVER_START_INDEX; row <= BOARD_SIZE; row++) {
                int index = getIndex(row, column, n);
                coverBoard[index][hBase] = true;
            }
        }
    }
    return hBase;
}

private int checkRowConstraint(boolean[][] coverBoard, int hBase) {
    for (int row = COVER_START_INDEX; row <= BOARD_SIZE; r++) {
        for (int n = COVER_START_INDEX; n <= BOARD_SIZE; n++, hBase++) {
            for (int column = COVER_START_INDEX; column <= BOARD_SIZE; column++) {
                int index = getIndex(row, column, n);
                coverBoard[index][hBase] = true;
            }
        }
    }
    return hBase;
}

private int checkCellConstraint(boolean[][] coverBoard, int hBase) {
    for (int row = COVER_START_INDEX; row <= BOARD_SIZE; row++) {
        for (int column = COVER_START_INDEX; column <= BOARD_SIZE; column++, hBase++) {
            for (int n = COVER_START_INDEX; n <= BOARD_SIZE; n++) {
                int index = getIndex(row, column, n);
                coverBoard[index][hBase] = true;
            }
        }
    }
    return hBase;
}

private boolean[][] initializeExactCoverBoard(int[][] board) {
    boolean[][] coverBoard = createExactCoverBoard();
    for (int row = COVER_START_INDEX; row <= BOARD_SIZE; row++) {
        for (int column = COVER_START_INDEX; column <= BOARD_SIZE; column++) {
            int n = board[row - 1][column - 1];
            if (n != NO_VALUE) {
                for (int num = MIN_VALUE; num <= MAX_VALUE; num++) {
                    if (num != n) {
                        Arrays.fill(coverBoard[getIndex(row, column, num)], false);
                    }
                }
            }
        }
    }
    return coverBoard;
}

node.prev.next = node.next
node.next.prev = node.prev

node.prev = node
node.next = node

class DancingNode {
    DancingNode L, R, U, D;
    ColumnNode C;

    DancingNode hookDown(DancingNode node) {
        assert (this.C == node.C);
        node.D = this.D;
        node.D.U = node;
        node.U = this;
        this.D = node;
        return node;
    }

    DancingNode hookRight(DancingNode node) {
        node.R = this.R;
        node.R.L = node;
        node.L = this;
        this.R = node;
        return node;
    }

    void unlinkLR() {
        this.L.R = this.R;
        this.R.L = this.L;
    }

    void relinkLR() {
        this.L.R = this.R.L = this;
    }

    void unlinkUD() {
        this.U.D = this.D;
        this.D.U = this.U;
    }

    void relinkUD() {
        this.U.D = this.D.U = this;
    }

    DancingNode() {
        L = R = U = D = this;
    }

    DancingNode(ColumnNode c) {
        this();
        C = c;
    }
}

class ColumnNode extends DancingNode {
    int size;
    String name;

    ColumnNode(String n) {
        super();
        size = 0;
        name = n;
        C = this;
    }

    void cover() {
        unlinkLR();
        for (DancingNode i = this.D; i != this; i = i.D) {
            for (DancingNode j = i.R; j != i; j = j.R) {
                j.unlinkUD();
                j.C.size--;
            }
        }
    }

    void uncover() {
        for (DancingNode i = this.U; i != this; i = i.U) {
            for (DancingNode j = i.L; j != i; j = j.L) {
                j.C.size++;
                j.relinkUD();
            }
        }
        relinkLR();
    }
}

private ColumnNode makeDLXBoard(boolean[][] grid) {
    int COLS = grid[0].length;

    ColumnNode headerNode = new ColumnNode("header");
    List<ColumnNode> columnNodes = new ArrayList<>();

    for (int i = 0; i < COLS; i++) {
        ColumnNode n = new ColumnNode(Integer.toString(i));
        columnNodes.add(n);
        headerNode = (ColumnNode) headerNode.hookRight(n);
    }
    headerNode = headerNode.R.C;

    for (boolean[] aGrid : grid) {
        DancingNode prev = null;
        for (int j = 0; j < COLS; j++) {
            if (aGrid[j]) {
                ColumnNode col = columnNodes.get(j);
                DancingNode newNode = new DancingNode(col);
                if (prev == null) prev = newNode;
                col.U.hookDown(newNode);
                prev = prev.hookRight(newNode);
                col.size++;
            }
        }
    }

    headerNode.size = COLS;

    return headerNode;
}

private ColumnNode selectColumnNodeHeuristic() {
    int min = Integer.MAX_VALUE;
    ColumnNode ret = null;
    for (
      ColumnNode c = (ColumnNode) header.R;
      c != header;
      c = (ColumnNode) c.R) {
        if (c.size < min) {
            min = c.size;
            ret = c;
        }
    }
    return ret;
}

private void search(int k) {
    if (header.R == header) {
        handleSolution(answer);
    } else {
        ColumnNode c = selectColumnNodeHeuristic();
        c.cover();

        for (DancingNode r = c.D; r != c; r = r.D) {
            answer.add(r);

            for (DancingNode j = r.R; j != r; j = j.R) {
                j.C.cover();
            }

            search(k + 1);

            r = answer.remove(answer.size() - 1);
            c = r.C;

            for (DancingNode j = r.L; j != r; j = j.L) {
                j.C.uncover();
            }
        }
        c.uncover();
    }
}