Query a list for only duplicates

Question

I have a List of type string in a .NET 3.5 project. The list has thousands of strings in it, but for the sake of brevity we're going to say that it just has 5 strings in it.

List<string> lstStr = new List<string>() {
            "Apple", "Banana", "Coconut", "Coconut", "Orange"};

Assume that the list is sorted (as you can tell above). What I need is a LINQ query that will remove all strings that are not duplicates. So the result would leave me with a list that only contains the two "Coconut" strings.

Is this possible to do with a LINQ query? If it is not then I'll have to resort to some complex for loops, which I can do, but I didn't want to unless I had to.

Answer 1

here is code for finding duplicates form string arrya

int[] listOfItems = new[] { 4, 2, 3, 1, 6, 4, 3 };
var duplicates = listOfItems
    .GroupBy(i => i)
    .Where(g => g.Count() > 1)
    .Select(g => g.Key);
foreach (var d in duplicates)
    Console.WriteLine(d);

Answer 2

var dupes = lstStr.Where(x => lstStr.Sum(y => y==x ? 1 : 0) > 1);

OR

var dupes = lstStr.Where((x,i) => (   (i > 0 && x==lstStr[i-1]) 
                                   || (i < lstStr.Count-1 && x==lstStr[i+1]));

Note that the first one enumerates the list for every element which takes O(n²) time (but doesn't assume a sorted list). The second one is O(n) (and assumes a sorted list).