This was an other fun one that didn’t take too much time. We are given to lists of integers inwhich we know that list 1 will have missing numbers from list 2. We need to find the numbers that are missing from list 1 and print them (on the same line) in ascending order.

input: 
10
203 204 205 206 207 208 203 204 205 206
13
203 204 204 205 206 207 205 208 203 206 205 206 204
output:
204 205 206

For this the first step was to create a map of the numbers and their counts.

    def map(list) do
        Enum.reduce(list, %{}, fn integer, map -> 
            Map.update(map, integer, 1, &(&1 + 1))
        end)
    end

This takes the elements of a list and returns the count of the numbers in a tuple setup.

Once we have the two maps we can then check the counts and append the any count difference to a list. Then because we need the list to be sorted… sort it.

    def find_difference([map1, map2]) do
        Enum.reduce(map2, [], fn {key, count2}, acc ->
          count1 = Map.get(map1, key, 0)
          if count2 > count1, do: [key | acc], else: acc
        end)
        |> Enum.sort()
    end

That is about it as I always leave the output to the user.