Brute force Algorithm for creation of Sudoku Board

Question

What I am developing is that initially the entire sudoku board is empty. One of the random cells(out of 81) is filled with a random value(1-9).

Now I want to fill all

Answer 1

Here's an implementation of the backtracking approach:

import java.util.Random;

public class Sudoku {

    public static void main(String[] args) {
        Random rand = new Random();
        int r = rand.nextInt(9);
        int c = rand.nextInt(9);
        int value = rand.nextInt(9) + 1;
        Board board = new Board();
        board.set(r, c, value);
        System.out.println(board);
        solve(board, 0);
        System.out.println(board);
    }

    private static boolean solve(Board board, int at) {
        if (at == 9*9)
            return true;
        int r = at / 9;
        int c = at % 9;
        if (board.isSet(r, c))
            return solve(board, at + 1);
        for (int value = 1; value <= 9; value++) {
            if (board.canSet(r, c, value)) {
                board.set(r, c, value);
                if (solve(board, at + 1))
                    return true;
                board.unSet(r, c);
            }
        }
        return false;
    }

    static class Board {
        private int[][] board = new int[9][9];
        private boolean[][] rs = new boolean[9][10];
        private boolean[][] cs = new boolean[9][10];
        private boolean[][][] bs = new boolean[3][3][10];
        public Board() {}
        public boolean canSet(int r, int c, int value) {
            return !isSet(r, c) && !rs[r][value] && !cs[c][value] && !bs[r/3][c/3][value];
        }
        public boolean isSet(int r, int c) {
            return board[r][c] != 0;
        }
        public void set(int r, int c, int value) {
            if (!canSet(r, c, value))
                throw new IllegalArgumentException();
            board[r][c] = value;
            rs[r][value] = cs[c][value] = bs[r/3][c/3][value] = true;
        }
        public void unSet(int r, int c) {
            if (isSet(r, c)) {
                int value = board[r][c];
                board[r][c] = 0;
                rs[r][value] = cs[c][value] = bs[r/3][c/3][value] = false;
            }
        }
        public String toString() {
            StringBuilder ret = new StringBuilder();
            for (int r = 0; r < 9; r++) {
                for (int c = 0; c < 9; c++)
                    ret.append(board[r][c]);
                ret.append("\n");
            }
            return ret.toString();
        }
    }
}