Q:

Convert List> into List in C#

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!

A:

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();

Update 09.2013
But these days I would actually write it as 
var result2 = l.SelectMany(i => i).Distinct();

A:

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

A:

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

