Let's assume my list is like this:

```
List<List<string>> zz = new List<List<string>>() { new List<string>{"1","2","3"}, new List<string> { "4", "5", "6" } };

```

I want one list as output as follows:

```
List<string> finalList = new List<string>{"1","2","3","4", "5", "6"};

```

**How do I flatten to this?**