How do I do an integer list intersection while keeping duplicates?

Question

I\'m working on a Greatest Common Factor and Least Common Multiple assignment and I have to list the common factors. Intersection() won\'t work because that removes duplicat

Answer 1

Are you looking for something like this? It should be pretty-much O(n+m), where n is the number of items in first and m is the number of items in second.

public static IEnumerable Overlap(this IEnumerable first,
    IEnumerable second, IEqualityComparer comparer = null)
{
    // argument checking, optimisations etc removed for brevity

    var dict = new Dictionary(comparer);

    foreach (T item in second)
    {
        int hits;
        dict.TryGetValue(item, out hits);
        dict[item] = hits + 1;
    }

    foreach (T item in first)
    {
        int hits;
        dict.TryGetValue(item, out hits);
        if (hits > 0)
        {
            yield return item;
            dict[item] = hits - 1;
        }
    }
}