## Saturday, 21 January 2017

### Chapter 22 Exercise 22, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

22.22 (Game: recursive Sudoku) Write a recursive solution for the Sudoku problem.

import java.util.Scanner;

public class Exercise22 {
public static void main(String[] args) {
int[][] grid = readAPuzzle();
if (!isValid(grid))
System.out.println("Invalid input");
else if (search(grid)) {
System.out.println("The solution is found:");
printGrid(grid);
} else
System.out.println("No solution");
}
/** Obtain a list of free cells from the puzzle */
public static int[][] getFreeCellList(int[][] grid) {
// Determine the number of free cells
int numberOfFreeCells = 0;
for (int i = 0; i < 9; i++)
for (int j = 0; j < 9; j++)
if (grid[i][j] == 0)
numberOfFreeCells++;
// Store free cell positions into freeCellList
int[][] freeCellList = new int[numberOfFreeCells][2];
int count = 0;
for (int i = 0; i < 9; i++)
for (int j = 0; j < 9; j++)
if (grid[i][j] == 0) {
freeCellList[count][0] = i;
freeCellList[count++][1] = j;
}
return freeCellList;
}

public static boolean search(int[][] grid) {
int[][] freeCellList = getFreeCellList(grid); // Free cells
if (freeCellList.length == 0)
return true; // "No free cells");
int k = 0; // Start from the first free cell
return search(grid, freeCellList, k);
}

/** Search for a solution */
public static boolean search(int[][] grid, int[][] freeCellList, int k) {
while (true) {
int i = freeCellList[k][0];
int j = freeCellList[k][1];
if (grid[i][j] == 0) {
grid[i][j] = 1;
}
if (isValid(i, j, grid)) {
if (k + 1 == freeCellList.length) {
return true;
} else if(search(grid, freeCellList, k + 1)) {
return true;
} else if (grid[i][j] == 9) {
grid[i][j] = 0;
return false;
} else {
grid[i][j]++;
}
} else if (grid[i][j] < 9) {
grid[i][j]++;
} else {
grid[i][j] = 0;
return false;
}
}
}

/** Check whether grid[i][j] is valid in the grid */
public static boolean isValid(int i, int j, int[][] grid) {
// Check whether grid[i][j] is valid at the i's row
for (int column = 0; column < 9; column++)
if (column != j && grid[i][column] == grid[i][j])
return false;
// Check whether grid[i][j] is valid at the j's column
for (int row = 0; row < 9; row++)
if (row != i && grid[row][j] == grid[i][j])
return false;
// Check whether grid[i][j] is valid in the 3 by 3 box
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 (row != i && col != j && grid[row][col] == grid[i][j])
return false;
return true; // The current value at grid[i][j] is valid
}
/** Print the values in the grid */
public static void printGrid(int[][] grid) {
for (int i = 0; i < 9; i++) {
for (int j = 0; j < 9; j++)
System.out.print(grid[i][j] + " ");
System.out.println();
}
}

/** Read a Sudoku puzzle from the keyboard */
public static int[][] readAPuzzle() {
// Create a Scanner
@SuppressWarnings("resource")
Scanner input = new Scanner(System.in);
System.out.println("Enter a Sudoku puzzle:");
int[][] grid = new int[9][9];
for (int i = 0; i < 9; i++)
for (int j = 0; j < 9; j++)
grid[i][j] = input.nextInt();
return grid;
}

/** Check whether the fixed cells are valid in the grid */
public static boolean isValid(int[][] grid) {
for (int i = 0; i < 9; i++)
for (int j = 0; j < 9; j++)
if (grid[i][j] < 0 || grid[i][j] > 9
|| (grid[i][j] != 0 && !isValid(i, j, grid)))
return false;
return true; // The fixed cells are valid
}
}