Use whatever language you like! Enjoy 
I’m curious to see how various languages allow one to express solutions to this problem.
I will post solutions of my own later.
This solution mutates the input. Python, nine lines.
Python Flatten
def flatten(val):
out = []
while val:
item = val.pop()
if isinstance(item, list):
val.extend(item)
elif item is not None:
out.append(item)
return list(reversed(out))
2 Likes
Pure function; the only memory use is in the output and the stack of iterators (which does not grow past the original nesting depth).
Python
def flatten(iterable):
stack = [iter(iterable)]
result = []
while stack:
for e in stack[-1]:
if isinstance(e, Iterable) and not isinstance(e, str):
stack.append(iter(e))
break
if e is not None:
result.append(e)
else:
stack.pop()
return result
My first attempt was just like MatthijsBlom’s stack idea, but after seeing IsaacG’s implicit stack, I came up with this:
Python
from collections.abc import Iterable
from itertools import chain
def flatten(iterable):
result = []
absent = object()
producer = iter(iterable)
while (val := next(producer, absent)) is not absent:
if isinstance(val, Iterable):
producer = chain(val, producer)
elif val is not None:
result.append(val)
return result
Unfortunately I couldn’t find a nicer way to iterate without try/except StopIteration. I woud appreciate any help from someone more experienced on how to clean it up.
UPD: Thanks to MatthijsBlom for suggestions on how to improve the original version.
That’s a great solution! I hadn’t thought to work on it in place during my turn. Thank you!