private boolean checkCol(char[][] board) {
        for (int i = 0; i < 9; i++) {
            HashSet<Character> set = new HashSet<Character>();
            for (int j = 0; j < 9; j++) {
                if (board[j][i] == '.') {
                    continue;
                }
                if (!set.contains(board[j][i])) {
                    set.add(board[j][i]);
                }
                else {
                    return false;
                }
            }
        }
        return true;
    }