data class User(val user_id:String = "",
val username:String = "",
val image:Int = 0,
var score:String = "")

val ref = FirebaseDatabase.getInstance().reference
ref.child("User").orderByChild("score").addListenerForSingleValueEvent(object : ValueEventListener {    
override fun onDataChange(userSnapshots : DataSnapshot) {        
    userSnapshots.children.forEach { snap ->          
    val mUser = snap.getValue(User::class.java) ?: return@forEach          
    tempList.add(mUser)      
 }
    }    
override fun onCancelled(e: DatabaseError) {    }
}