now i feel stupid, good one

best one

That's clever...

Oh, yeah. thx

Great job!
Just FYI,

.sorted((a, b) -> { return a.replaceAll("\\D+", "").compareTo(b.replaceAll("\\D+", "")); })

could be replaced with

.sorted(Comparator.comparing(a -> a.replaceAll("\\D+", "")))

No random tests

The squareroot should be save into a variable, instead of being calculated twice.

Test cases assume a certain ordering of Set(HashSet) objects, which should not be done, either replace with SortedSet(TreeSet) or remove entirely.

Using a stack that explodes each list as it is encounters is a great solution.

I also like the use of "instanceof Collection" here instead of checking if it's a List or Set.

Duplicate:
https://www.codewars.com/kata/flatten
https://www.codewars.com/kata/flattening-lists

FYI - typo in tags algoritms

Fun Kata. Thanks!

Hi, thanks for your response and fixing the testcase;-)!

Hi, seems like i made an error in one of my Testcases. It should be fixed now though, you can try again if you like. Sorry for the inconvenience.

Hi, are your (submit) testcases correct? See no problem with my code, tested deeper lists too without errors (null and empty lists too), but submit directly throws error (first case, long list):

  testDeepFlattening(CollectionUtilsTest)
✘ Runtime Error Occurred
 Log
 java.lang.UnsupportedOperationException
	at java.util.AbstractList.add(AbstractList.java:148)
	at 
  :
  : (long list)

A testoutput like 'System.out.println("Test"); return "emptyList"' (without other code) directly throws same error without output of "Test"...