Hackerrank Collections and Data Structures 2

Question 1

What does `collections.OrderedDict` guarantee that a regular `dict` does NOT guarantee in Python versions before 3.7?

Accepted Answer

Insertion order preservation

Answer

OrderedDict preserves the insertion order of keys, which regular dicts did not guarantee before Python 3.7.

Question 2

Which method of `collections.deque` removes and returns an element from the LEFT end?

Accepted Answer

popleft()

Answer

`popleft()` removes and returns the leftmost element of a deque in O(1) time.

Question 3

What is the output of: `from collections import Counter; c = Counter('aabbc'); print(c.most_common(2))`?

Accepted Answer

[('a', 2), ('b', 2)]

Answer

`most_common(2)` returns the two most frequent elements as a list of (element, count) tuples; ties are ordered arbitrarily.

Question 4

What happens when you access a missing key in a `collections.defaultdict(list)`?

Accepted Answer

Creates an entry with an empty list

Answer

defaultdict calls its factory function (list) to create a default value for missing keys, inserting an empty list.

Question 5

Which named tuple method returns a new instance replacing specified fields with new values?

Accepted Answer

_replace()

Answer

`_replace()` returns a new named tuple instance with specified fields replaced by new values.

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