"""

Flatten Arrays

Given an array that may contain nested arrays, produce a single

flat resultant array.

Reference: https://en.wikipedia.org/wiki/Flatten_(higher-order_function)

Complexity:

Time: O(n) where n is the total number of elements

Space: O(n)

"""

from __future__ import annotations

from collections.abc import Generator, Iterable

from typing import Any

def flatten(input_arr: Iterable[Any], output_arr: list[Any] | None = None) -> list[Any]:

"""Recursively flatten a nested iterable into a single list.

Args:

input_arr: A potentially nested iterable to flatten.

output_arr: Accumulator list for recursive calls (internal use).

Returns:

A flat list containing all leaf elements.

Examples:

>>> flatten([2, 1, [3, [4, 5], 6], 7, [8]])

[2, 1, 3, 4, 5, 6, 7, 8]

"""

if output_arr is None:

output_arr = []

for element in input_arr:

if not isinstance(element, str) and isinstance(element, Iterable):

flatten(element, output_arr)

else:

output_arr.append(element)

return output_arr

def flatten_iter(iterable: Iterable[Any]) -> Generator[Any, None, None]:

"""Lazily flatten a nested iterable, yielding one element at a time.

Args:

iterable: A potentially nested iterable to flatten.

Returns:

A generator producing one-dimensional output.

Examples:

>>> list(flatten_iter([2, 1, [3, [4, 5], 6], 7, [8]]))

[2, 1, 3, 4, 5, 6, 7, 8]

"""

for element in iterable:

if not isinstance(element, str) and isinstance(element, Iterable):

yield from flatten_iter(element)

else:

yield element