Sudoku Solver

SudokuSolver.java

public class SudokuSolver {
	private static final int GRID_SIZE = 9;
	private static final int BOX_SIZE = 3;

	public static void main(String[] args) {

		int[][] board = { 
				{ 7, 0, 2, 0, 5, 0, 6, 0, 0 }, 
				{ 0, 0, 0, 0, 0, 3, 0, 0, 0 }, 
				{ 1, 0, 0, 0, 0, 9, 5, 0, 0 },
				{ 8, 0, 0, 0, 0, 0, 0, 9, 0 }, 
				{ 0, 4, 3, 0, 0, 0, 7, 5, 0 }, 
				{ 0, 9, 0, 0, 0, 0, 0, 0, 8 },
				{ 0, 0, 9, 7, 0, 0, 0, 0, 5 }, 
				{ 0, 0, 0, 2, 0, 0, 0, 0, 0 }, 
				{ 0, 0, 7, 0, 4, 0, 2, 0, 3 }
				};
		System.out.println("\nInput ..\n");
		printBoard(board);
		
		if (solveBoard(board)) {
			System.out.println("\nSolved !!\n");
			printBoard(board);
		} else {
			System.out.println("\nUnsolvable !!");
		}
	}

	private static void printBoard(int[][] board) {
		for (int row = 0; row < GRID_SIZE; row++) {
			if (row % 3 == 0 && row != 0) {
				System.out.println("------------");
			}
			for (int column = 0; column < GRID_SIZE; column++) {
				if (column % 3 == 0 && column != 0) {
					System.out.print("|");
				}
				System.out.print(board[row][column]);
			}
			System.out.println();
		}
	}

	private static boolean isNumberInRow(int[][] board, int number, int row) {
		for (int i = 0; i < GRID_SIZE; i++) {
			if (board[row][i] == number) {
				return true;
			}
		}
		return false;
	}

	private static boolean isNumberInColumn(int[][] board, int number, int column) {
		for (int i = 0; i < GRID_SIZE; i++) {
			if (board[i][column] == number) {
				return true;
			}
		}
		return false;
	}

	private static boolean isNumberInBox(int[][] board, int number, int row, int column) {
		int initialBoxRow = row - row % BOX_SIZE;
		int initialBoxColumn = column - column % BOX_SIZE;

		for (int i = initialBoxRow; i < initialBoxRow + BOX_SIZE; i++) {
			for (int j = initialBoxColumn; j < initialBoxColumn + BOX_SIZE; j++) {
				if (board[i][j] == number) {
					return true;
				}
			}
		}
		return false;
	}

	private static boolean isValidPlacement(int[][] board, int number, int row, int column) {
		return !isNumberInRow(board, number, row) && !isNumberInColumn(board, number, column)
				&& !isNumberInBox(board, number, row, column);
	}

	private static boolean solveBoard(int[][] board) {
		for (int row = 0; row < GRID_SIZE; row++) {
			for (int column = 0; column < GRID_SIZE; column++) {
				if (board[row][column] == 0) {
					for (int numberToTry = 1; numberToTry <= GRID_SIZE; numberToTry++) {
						if (isValidPlacement(board, numberToTry, row, column)) {
							board[row][column] = numberToTry;
							if (solveBoard(board)) {
								return true;
							} else {
								board[row][column] = 0;
							}
						}
					}
					return false;
				} 
			}
		}
		return true;
	}
}