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public class Solution { public static void main(String[] args) { Solution s = new Solution(); char[][] board = {{'.', '2', '6', '5', '.', '.', '.', '9', '.'}, {'5', '.', '.', '.', '7', '9', '.', '.', '4'}, {'3', '.', '.', '.', '1', '.', '.', '.', '.'}, {'6', '.', '.', '.', '.', '.', '8', '.', '7'}, {'.', '7', '5', '.', '2', '.', '.', '1', '.'}, {'.', '1', '.', '.', '.', '.', '4', '.', '.'}, {'.', '.', '.', '3', '.', '8', '9', '.', '2'}, {'7', '.', '.', '.', '6', '.', '.', '4', '.'}, {'.', '3', '.', '2', '.', '.', '1', '.', '.'}}; s.solver(board); } public boolean solver(char[][] board) { for (int r = 0; r < board.length; r++) { for (int c = 0; c < board[0].length; c++) { if (board[r][c] == '.') { for (int k = 1; k <= 9; k++) { board[r][c] = (char) ('0' + k); if (isValid(board, r, c) && solver(board)) { return true; } else { board[r][c] = '.'; } } return false; } } } return true; } public boolean isValid(char[][] board, int r, int c) { //check row boolean[] row = new boolean[9]; for (int i = 0; i < 9; i++) { if (board[r][i] >= '1' && board[r][i] <= '9') { if (row[board[r][i] - '1'] == false) { row[board[r][i] - '1'] = true; } else { return false; } } } //check column boolean[] col = new boolean[9]; for (int i = 0; i < 9; i++) { if (board[i][c] >= '1' && board[i][c] <= '9') { if (col[board[i][c] - '1'] == false) { col[board[i][c] - '1'] = true; } else { return false; } } } //check the 3*3 grid boolean[] grid = new boolean[9]; for (int i = (r / 3) * 3; i < (r / 3) * 3 + 3; i++) { for (int j = (c / 3) * 3; j < (c / 3) * 3 + 3; j++) { if (board[i][j] >= '1' && board[i][j] <= '9') { if (grid[board[i][j] - '1'] == false) { grid[board[i][j] - '1'] = true; } else { return false; } } } } retu