1 · Model, solve and test · 25 MIN

Consume a generator without losing order

Consume a generator without losing order

A generator yields values on demand and resumes from its last suspension point. This flatten generator visits pages and then each item within a page, preserving the original encounter order. The final list deliberately materializes the result because the exercise asks for a JSON array. Distinguish the generator’s small working state from that returned list: a streaming producer does not make a fully materialized result constant-space.

Use yield from for each inner iterable, then materialize only at the output boundary.

Read the example

def flatten(pages):
    for page in pages:
        yield from page
def solve(input):
    return list(flatten(input))
import json
print(json.dumps(solve(json.loads("[[1,2],[],[3]]")), separators=(",", ":")))

[1,2,3]

Your challenge

Given a list of pages, each a list of numbers, return one flat list preserving page order and item order. Empty pages contribute no items.

Solution cost: O(p + n) for p pages and n items. time · O(n) returned list; constant extra generator state. space

Common trap

Calling list(generator) consumes the generator; a second iteration will not replay its values.