Convert List<List<T>> into List<T> in C#

Question

I have a List<List<int>>. I would like to convert it into a List<int> where each int is unique. I was wondering if anyone had an elegant solution to this using LINQ.

I would like to be able to use the Union method but it creates a new List<> everytime. So I'd like to avoid doing something like this:

List<int> allInts = new List<int>();

foreach(List<int> list in listOfLists)
   allInts = new List<int>(allInts.Union(list));

Any suggestions?

Thanks!

Answer 1

How about:

HashSet<int> set = new HashSet<int>();
foreach (List<int> list in listOfLists)
{
    set.UnionWith(list);
}
return set.ToList();

Answer 2

List<List<int>> l = new List<List<int>>();

l.Add(new List<int> { 1, 2, 3, 4, 5, 6});
l.Add(new List<int> { 4, 5, 6, 7, 8, 9 });
l.Add(new List<int> { 8, 9, 10, 11, 12, 13 });

var result = (from e in l
              from e2 in e
              select e2).Distinct();

Answer 3

List<int> result = listOfLists
  .SelectMany(list => list)
  .Distinct()
  .ToList();