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/*Java Program to solve Sudoku problem using Backtracking*/
public class Main {
public static boolean isValid(int[][] puzzle, int row, int col, int number) {
// Row has the unique
for (int d = 0; d < puzzle.length; d++) {
/* If the number we are trying to
place is already present in
that row, then return false;*/
if (puzzle[row][d] == number) {
return false;
}
}
// Column has the unique numberbers
for (int r = 0; r < puzzle.length; r++) {
/* If the number we are trying to
place is already present in
that column, then return false;*/
if (puzzle[r][col] == number) {
return false;
}
}
//Corresponding square has unique number
int sqrt = (int) Math.sqrt(puzzle.length);
int bRow = row - row % sqrt;
int bCol = col - col % sqrt;
for (int r = bRow; r < bRow + sqrt; r++) {
for (int d = bCol; d < bCol + sqrt; d++) {
if (puzzle[r][d] == number) {
return false;
}
}
}
// if there is no clash, it's safe
return true;
}
public static boolean solution(int[][] puzzle, int n) {
int row = -1;
int col = -1;
boolean isEmpty = true;
for (int i = 0; i < n; i++) {
for (int j = 0; j < n; j++) {
if (puzzle[i][j] == 0) {
row = i;
col = j;
isEmpty = false;
break;
}
}
if (!isEmpty) {
break;
}
}
// No empty space left
if (isEmpty) {
return true;
}
// Else backtrack for each-row
for (int number = 1; number <= n; number++) {
if (isValid(puzzle, row, col, number)) {
puzzle[row][col] = number;
if (solution(puzzle, n)) {
// print(puzzle, n);
return true;
} else {
// replace it
puzzle[row][col] = 0;
}
}
}
return false;
}
//To Print the Sudoku
public static void printSudoku(int[][] puzzle, int N) {
for (int r = 0; r < N; r++) {
for (int d = 0; d < N; d++) {
System.out.print(puzzle[r][d]);
System.out.print(" ");
}
System.out.print("\n");
if ((r + 1) % (int) Math.sqrt(N) == 0) {
System.out.print("");
}
}
}
// Driver Code
public static void main(String args[]) {
//Here 0 represents the unassigned value
int[][] puzzle = new int[][] {
{ 3, 0, 5, 4, 0, 2, 0, 6, 0 },
{ 4, 9, 0, 7, 6, 0, 1, 0, 8 },
{ 6, 0, 0, 1, 0, 3, 2, 4, 5 },
{ 0, 0, 3, 9, 0, 0, 5, 8, 0 },
{ 9, 6, 0, 0, 5, 8, 7, 0, 3 },
{ 0, 8, 1, 3, 0, 4, 0, 9, 2 },
{ 0, 5, 0, 6, 0, 1, 4, 0, 0 },
{ 2, 0, 0, 5, 4, 9, 0, 7, 0 },
{ 1, 4, 9, 0, 0, 7, 3, 0, 6 }
};
int N = puzzle.length;
System.out.println("Before Solving");
printSudoku(puzzle, N);
System.out.println("");
System.out.println("After Solving");
if (solution(puzzle, N)) {
printSudoku(puzzle, N);
} else {
System.out.println("No solution exists");
}
}
}