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

• Find the intersection of the two lists.
• Group the lists by the intersecting items
• Join the groups, and select the Min(Count) for each item
• Flatten into a new list.

See below:

var intersect = list1.Intersect(list2).ToList();
var groups1 = list1.Where(e => intersect.Contains(e)).GroupBy(e => e);
var groups2 = list2.Where(e => intersect.Contains(e)).GroupBy(e => e);

var allGroups = groups1.Concat(groups2);

return allGroups.GroupBy(e => e.Key)
    .SelectMany(group => group
        .First(g => g.Count() == group.Min(g1 => g1.Count())))
    .ToList();