code to delete the row if the cells on specific column are unique - vba

Question

What I am trying to achieve is to create a vba code to completely delete the rows if the value in column C (Id) is unique. So in example below the rows 6 and 7 would be deleted since the 111115 and 111116 are not showing up more than once in this column C. Any help is welcome! Thanks a lot.
Code so far: (but not working yet)
Sub delete_not_duplicates()
Dim i As Integer, j As Integer, toDel As Boolean, theNum As Integer
i = 2
Do While Cells(i, 3).Value <> ""
toDel = True
theNum = Cells(i, 3).Value
Do While Cells(j, 3).Value <> ""
If i <> j and Cells(j, 3) == theNum Then
toDel = False
Loop
If toDel == true Then
Rows(i).Delete
Else
i = i + 1
End If
Loop
End Sub

Answer 1

The general approach to do do this in a reasonable fast way is to
Get your data into a Variant Array
Loop the array, identifying unique values
Build a range reference to rows to be deleted, but defer the deletion
After the loop, delete all rows in one go
Sub demo()
Dim rDel As Range, rng As Range
Dim dat As Variant
Dim i As Long, cnt As Long
Dim TestCol As Long
' Avoid magic numbers
TestCol = 3 ' Column C
' Reference the correct sheet
With ActiveSheet
' Get data range
Set rng = .Range(.Cells(1, TestCol), .Cells(.Rows.Count, TestCol).End(xlUp))
' Get data as a Variant Array to speed things up
dat = rng.Value
' Loop the Variant Array
For i = 2 To UBound(dat, 1)
' Is value unique?
cnt = Application.CountIfs(rng, dat(i, 1))
If cnt = 1 Then
' If so, add to delete range
If rDel Is Nothing Then
Set rDel = .Cells(i, TestCol)
Else
Set rDel = Union(rDel, .Cells(i, TestCol))
End If
End If
Next
End With
' Do the delete
If Not rDel Is Nothing Then
rDel.EntireRow.Delete
End If
End Sub

Answer 2

I think the most efficient way would be:
Initialize an empty HashSet< Integer> (or whatever generic type you want) which will represent all the unique entries of C (id), let's name it uniqueIdSet
Iterate through the 2D array
if(uniqueIdSet.contains(id)){
//if the id was already seen before, it means it's not unique
uniqueIdSet.remove(id);
}
else{
//we haven't seen this id yet, add it to the unique set
uniqueIdSet.add(id);
}
Iterate through the original array again and do:
if(uniqueSet.contains(id)){
//if the id is unique, remove it from the array.
array.remove(currentRow);
}
Depending on your implementation, you might not be able to remove from the array as you iterate through it. A way around it is initializing a copy of the original array and remove the respective row from there.