Remove List Elements that appear more than Once In Place

Question

There is a similar question posted, but I do not have the rep to ask a follow-up question in that thread :(

That question and solution is HERE.

If I have a Lis

Answer 1

So, if you can have a new list, then this is the easiest way to do it:

var source = new List<int>() { 4, 5, 7, 3, 5, 4, 2, 4 };

var result =
    source
        .GroupBy(x => x)
        .Where(x => !x.Skip(1).Any())
        .Select(x => x.Key)
        .ToList();

This gives:

{ 7, 3, 2 }

If you want to remove the values from the original source, then do this:

var duplicates =
    new HashSet<int>(
        source
            .GroupBy(x => x)
            .Where(x => x.Skip(1).Any())
            .Select(x => x.Key));

source.RemoveAll(n => duplicates.Contains(n));

Answer 2

I have two options for you, one that uses HashSet and other Linq .

Option 1:

Using HashSet, loop through collection and insert if it not exist and remove if it exists.

HashSet<int> hash = new HashSet<int>();

foreach(var number in list)
{
    if(!hash.Contains(number)) hash.Add(number);
    else hash.Remove(number);               
}
list = hash.ToList();

Option 2:

Simple Linq, group the elements and filter whose count >1.

var list= list.GroupBy(g=>g)
    .Where(e=>e.Count()==1)
    .Select(g=>g.Key)
    .ToList();

There is big performance gain using HashSet over Linq, it is obvious, Linq(in this case) require multiple iterations, where as HashSet uses single iteration and provides LookUp (for adding/removing) with O(1) access.

Elapsed Time (Using Linq): 8808 Ticks
Elapsed Time (Using HashSet): 51 Ticks

Working Demo