var ref: DatabaseReference!

tableView.delegate = self
    tableView.dataSource = self
    ref = Database.database().reference()

    var i : Int = -1
    ref.child(selectedNo!).observeSingleEvent(of: .value, with: { (snapshot) in
        for folder in snapshot.children {
            i += 1
            if let snap = folder as? DataSnapshot {
                let dic = snap.value as! NSDictionary
                self.elements.append(dic["image"] as! String)
            }
        }
        self.tableView.reloadData()
    }) { (error) in
        print(error.localizedDescription)
    }