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SudokuSolver.java
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SudokuSolver.java
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public class SudokuSolver {
// board size
static final int ROW_LEN = 9;
static final int COL_LEN = 9;
public void solveSudoku(char[][] board) {
if (board == null || board.length == 0 || board[0].length == 0) {
throw new IllegalArgumentException("Input is empty");
}
boolean[][] rowVisited = new boolean[ROW_LEN][COL_LEN + 1];
boolean[][] colVisited = new boolean[ROW_LEN][COL_LEN + 1];
boolean[][] blockVisited = new boolean[ROW_LEN][COL_LEN + 1];
initialize(board, rowVisited, colVisited, blockVisited);
fillOutSudoku(board, 0, rowVisited, colVisited, blockVisited);
}
private boolean fillOutSudoku(char[][] board, int position, boolean[][] rowVisited, boolean[][] colVisited,
boolean[][] blockVisited) {
if (position == 81) return true;
int row = position / COL_LEN;
int col = position % COL_LEN;
if (board[row][col] == '.') {
for (int num = 1; num <= 9; num++) {
if (canFill(row, col, num, rowVisited, colVisited, blockVisited)) {
System.out.println("[" + row + ", " + col + ", " + position + "]" + "num = " + (char) (num + '0'));
board[row][col] = (char) (num + '0');
updateVisitedStatus(num, row, col, rowVisited, colVisited, blockVisited, /* isMarkVisited= */true);
if (fillOutSudoku(board, position + 1, rowVisited, colVisited, blockVisited)) {
return true;
}
board[row][col] = '.';
updateVisitedStatus(num, row, col, rowVisited, colVisited, blockVisited, /* isMarkVisited= */false);
}
}
} else if (fillOutSudoku(board, position + 1, rowVisited, colVisited, blockVisited)) {
return true;
}
return false;
}
private void updateVisitedStatus(int num, int row, int col, boolean[][] rowVisited, boolean[][] colVisited, boolean[][] blockVisited, boolean isMarkVisited) {
rowVisited[row][num] = isMarkVisited;
colVisited[col][num] = isMarkVisited;
// The block position is row / 3 + col.
blockVisited[row / 3 * 3 + col / 3][num] = isMarkVisited;
}
private boolean canFill(int row, int col, int num, boolean[][] rowVisited, boolean[][] colVisited, boolean[][] blockVisited) {
if (rowVisited[row][num]) {
return false;
}
if (colVisited[col][num]) {
return false;
}
// The block position is row / 3 * 3 + col / 3.
if (blockVisited[row / 3 * 3 + col / 3][num]) {
return false;
}
return true;
}
private void initialize(char[][] board, boolean[][] rowVisited, boolean[][] colVisited, boolean[][] blockVisited) {
for (int row = 0; row < ROW_LEN; row++) {
for (int col = 0; col < COL_LEN; col++) {
if (board[row][col] != '.') {
if (!Character.isDigit(board[row][col])) {
throw new IllegalArgumentException("The character should be digit, but it is " + board[row][col]);
}
int num = board[row][col] - '0';
rowVisited[row][num] = true;
colVisited[col][num] = true;
// The block position is row / 3 * 3 + col / 3.
blockVisited[row / 3 * 3 + col / 3][num] = true;
}
}
}
}
public static void main(String[] args) {
SudokuSolver sudokuSolver = new SudokuSolver();
char[][] board =
{
{'5','3','.','.','7','.','.','.','.'},
{'6','.','.','1','9','5','.','.','.'},
{'.','9','8','.','.','.','.','6','.'},
{'8','.','.','.','6','.','.','.','3'},
{'4','.','.','8','.','3','.','.','1'},
{'7','.','.','.','2','.','.','.','6'},
{'.','6','.','.','.','.','2','8','.'},
{'.','.','.','4','1','9','.','.','5'},
{'.','.','.','.','8','.','.','7','9'}};
sudokuSolver.solveSudoku(board);
for (int i = 0; i < 9; i++) {
for (int j = 0; j < 9; j++) {
System.out.print(board[i][j] + " ");
}
System.out.println();
}
}
}