I created a dictionary with the number and the number of times it was listed.
Then checked if any of the numbers in the 2nd list were in that dictionary.
What was left of the dictionary after comparison is the answer !

The description of the task doesn't mention that the lists are only comprised of numbers but the tests only check for numbers. Would be clearer if that was pointed out in the task.

Loved this Kata, great for teaching to use a stream

Popular kata just got even more popular
Translation Dart

Thank you! i used a map and the time complexity is O(N).

ye definately it is not 6.. its too easy.

kata hint != kata suggestion

if you use a iteration to comprobe if b include element from a is better to do the iteration in descending order to dont have error

this kata seemed much easier than some of the 7kyu katas

should be fixed (made description language agnostic in the meantime

The bear effect

Not a kata suggestion. A lot of katas have very biased upvotes, since the top-voted solutions are always at the top, they get upvoted constantly, but that does not mean they are "best practices".