Expected O(n) time solution without using the built-in sort function.

Not enforced.

Traceback (most recent call last):
  File "/runner/frameworks/python/cw-2.py", line 111, in wrapper
    func()
  File "main.py", line 123, in test_everything
    test.assert_equals(_ref(nums[:]), partially_sorted(nums[:]))
  File "main.py", line 89, in _ref
    counts = Counter((nums[n] & 0xF for n in range(i, j)))
NameError: name 'Counter' is not defined