Longest list in SortedList of Lists

Question

I have a SortedList of Lists and I am interested in finding the KEY that corresponds to the longest list (list with the most items in it). In code, that looks like:

// how the list is defined:
var myList = new SortedList<long, List<string>>();

// EXAMPLE data only:
myList.Add(0, new List<string>());
myList[0].AddRange(new []{"a", "b", "c"});

myList.Add(8, new List<string>());
myList[8].AddRange(new []{"1", "2"});

myList.Add(23, new List<string>());
myList[23].AddRange(new []{"c", "d", "e", "f", "g"});

In the above example the result should be "23" since that is the key that goes with the longest list.

I know how to write this with a for loop, but I think this should be a simple to do with LINQ. That said, I can't seem to get the syntax quite right! Any help is appreciated!

Answer 1

There's maybe a more efficient way, but you can order by count (of value) descending, and take first.

myList.OrderByDescending(m => m.Value.Count()).First().Key;

of course, if you want all the keys with highest count (they may be multiple values with same length), you should do a group by count.

Something like that.

myList.GroupBy(m => m.Value.Count())
      .OrderByDescending(m => m.Key)//I'm the key of the group by
      .First()
      .Select(g => g.Key);//I'm the key of the SortedList

So if you add to your sample an item with same list length

myList.Add(24, new List<string>());
myList[24].AddRange(new[] {"a", "b", "c", "d", "e"});

you will get 23 And 24.

same could be achieved with

from item in myList
let maxCount = myList.Max(x => x.Value.Count())
where item.Value.Count() == maxCount
select item.Key;

Answer 2

Whilst a sort will give you correct results, it requires O(n log n) time to execute, which is asymptotically higher than a simple O(n) sweep:

int maxLength = myList.Max(x => x.Value.Count);
var longestKeys = myList.Where(x => x.Value.Count == maxLength).Select(x => x.Key);