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Sudoku.java
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Sudoku.java
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package com.thealgorithms.others;
final class Sudoku {
private Sudoku() {
}
public static boolean isSafe(int[][] board, int row, int col, int num) {
// Row has the unique (row-clash)
for (int d = 0; d < board.length; d++) {
// Check if the number we are trying to
// place is already present in
// that row, return false;
if (board[row][d] == num) {
return false;
}
}
// Column has the unique numbers (column-clash)
for (int r = 0; r < board.length; r++) {
// Check if the number
// we are trying to
// place is already present in
// that column, return false;
if (board[r][col] == num) {
return false;
}
}
// Corresponding square has
// unique number (box-clash)
int sqrt = (int) Math.sqrt(board.length);
int boxRowStart = row - row % sqrt;
int boxColStart = col - col % sqrt;
for (int r = boxRowStart; r < boxRowStart + sqrt; r++) {
for (int d = boxColStart; d < boxColStart + sqrt; d++) {
if (board[r][d] == num) {
return false;
}
}
}
// if there is no clash, it's safe
return true;
}
public static boolean solveSudoku(int[][] board, 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 (board[i][j] == 0) {
row = i;
col = j;
// We still have some remaining
// missing values in Sudoku
isEmpty = false;
break;
}
}
if (!isEmpty) {
break;
}
}
// No empty space left
if (isEmpty) {
return true;
}
// Else for each-row backtrack
for (int num = 1; num <= n; num++) {
if (isSafe(board, row, col, num)) {
board[row][col] = num;
if (solveSudoku(board, n)) {
// print(board, n);
return true;
} else {
// replace it
board[row][col] = 0;
}
}
}
return false;
}
public static void print(int[][] board, int n) {
// We got the answer, just print it
for (int r = 0; r < n; r++) {
for (int d = 0; d < n; d++) {
System.out.print(board[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) {
int[][] board = new int[][] {
{3, 0, 6, 5, 0, 8, 4, 0, 0},
{5, 2, 0, 0, 0, 0, 0, 0, 0},
{0, 8, 7, 0, 0, 0, 0, 3, 1},
{0, 0, 3, 0, 1, 0, 0, 8, 0},
{9, 0, 0, 8, 6, 3, 0, 0, 5},
{0, 5, 0, 0, 9, 0, 6, 0, 0},
{1, 3, 0, 0, 0, 0, 2, 5, 0},
{0, 0, 0, 0, 0, 0, 0, 7, 4},
{0, 0, 5, 2, 0, 6, 3, 0, 0},
};
int n = board.length;
if (solveSudoku(board, n)) {
// print solution
print(board, n);
} else {
System.out.println("No solution");
}
}
}