get common elements in lists in C#

Question

I have two sorted lists as below:

var list1 = new List<int>() { 1, 1, 1, 2, 3 };
var list2 = new List<int>() { 1, 1, 2, 2, 4 };

I want the output to be: {1, 1, 2}

How to do this in C#? Is there a way using Linq?

Answer 1

The extra 1 means you can't use Intersect because it returns a set.

Here's some code that does what you need:

var list1 = new List<int>() { 1, 1, 1, 2, 3 };
var list2 = new List<int>() { 1, 1, 2, 2, 4 };

var grouped1 =
    from n in list1
    group n by n
    into g
    select new {g.Key, Count = g.Count()};

var grouped2 =
    from n in list2
    group n by n
    into g
    select new {g.Key, Count = g.Count()};

var joined =
    from b in grouped2
    join a in grouped1 on b.Key equals a.Key
    select new {b.Key, Count = Math.Min(b.Count, a.Count)};

var result = joined.SelectMany(a => Enumerable.Repeat(a.Key, a.Count));

CollectionAssert.AreEquivalent(new[] {1, 1, 2}, result);

Answer 2

While both @Austin Salonen's solution and @Enigmativity's solution work for any given lists, neither take advantage of OP's condition that the lists are sorted.

Given that both lists will be ordered we can do a search in O(n + m) time where n and m are the length of each list. Not entirely sure what the previous solutions big o performance is, but it's definitely slower then O(n + m).

Basically we just walk both lists, moving one or both enumerators based on a comparison check.

var results = new List<int>();
var e1 = list1.GetEnumerator();
var e2 = list2.GetEnumerator();
var hasNext = e1.MoveNext() && e2.MoveNext();

while (hasNext) {
    var value1 = e1.Current;
    var value2 = e2.Current;

    if (value1 == value2) {
        results.Add(value1);
        hasNext = e1.MoveNext() && e2.MoveNext();
    } else if (value1 < value2) {
        hasNext = e1.MoveNext();
    } else if (value1 > value2) {
        hasNext = e2.MoveNext();
    }
}

That's it! results will be an empty list if no matches are found. Note this assumes both lists are in ascending order. If it's descending, just flip the < and > operators.

Answer 3

Use Intersect:

 var commonElements = list1.Intersect(list2).ToList();

Answer 4

I am late in answering this question, this might help future visitors.

            List<int> p = new List<int> { 1, 1, 1, 2, 3 };
            List<int> q = new List<int> { 1, 1, 2, 2, 4 };
            List<int> x = new List<int>();
            for (int i = 0; i < p.Count; i++ )
            {
                if (p[i] == q[i])
                {
                    x.Add(p[i]);
                }
            }

Answer 5

This works nicely:

var list1 = new List<int>() { 1, 1, 1, 2, 3 };
var list2 = new List<int>() { 1, 1, 2, 2, 4 };

var lookup1 = list1.ToLookup(x => x);
var lookup2 = list2.ToLookup(x => x);

var results = lookup1.SelectMany(l1s => lookup2[l1s.Key].Zip(l1s, (l2, l1) => l1));