Gave it another shot: def monotonic(arr): if not arr: return [] new_arr = [] sub_arr = [arr[0]] parity = 0 for n

Gave it another shot: def monotonic(arr): if not arr: return [] new_arr = [] sub_arr = [arr[0]] parity = 0 for n in arr[1:]: delta = n - sub_arr[-1] if parity == 0: parity += delta elif parity * delta < 0: new_arr.append(sorted(sub_arr)) sub_arr = [] parity = 0 sub_arr.append(n) new_arr.append(sorted(sub_arr)) return new_arr `delta` keeps track of whether the next element increases or decreases, whereas `parity` keeps track of the parity of the run. No imports, arbitrary iterators, single function. You can continue to pare it down.