data_structures: add progressive_set_intersection in disjoint_set

[Devanik21]

Description

This PR adds progressive_set_intersection() to the repository.

Problem it addresses

Python's built-in set.intersection(*others) is already highly optimized in C and automatically iterates over the smallest set. However, when intersecting many sets (50–100+) or dealing with highly imbalanced sizes (e.g., one tiny set of 10 elements vs. several sets with millions of elements), a naive approach can waste time.

This implementation demonstrates the "smallest-first + progressive pruning" heuristic:

• Sort all input sets by size (ascending).
• Start with a copy of the smallest set.
• Progressively intersect with the remaining sets.
• Early exit as soon as the result becomes empty.

This pattern significantly reduces unnecessary membership checks in practice.

Why add this to the repo?

• Educational value: Clearly shows an important optimization technique for multi-set operations.
• Pure Python, zero external dependencies.
• Works with any hashable elements (no assumptions about integer ranges or sorting).
• Includes comprehensive doctests.
• Handles edge cases: empty input, single set, empty intersection.

Note: For most everyday use cases, the built-in set.intersection() remains the best choice. This module is primarily for learning and for scenarios with many/imbalanced sets.

Algorithm Details

• Time complexity: In the worst case ~O(min_size × k) where k is the number of sets, but much faster in practice due to early pruning.
• Space complexity: O(min_size) extra space (copy of the smallest set).

Related Issue

Closes #14368

Files Changed

• data_structures/disjoint_set/progressive_set_intersection.py (new file)

Testing

• All doctests pass (python3 -m doctest -v progressive_set_intersection.py)
• Follows repo style (snake_case, proper docstring, type hints where appropriate)

Example Usage

from data_structures.disjoint_set.progressive_set_intersection import progressive_set_intersection

s1 = {1, 2, 3, 4}
s2 = {2, 3, 5, 6}
s3 = {2, 3, 7}

result = progressive_set_intersection(s1, s2, s3)
print(result)          # Output: {2, 3}

---

Would be happy to add more functions (e.g., sorted array intersection using two-pointer technique) or a bitmap version in a follow-up PR if needed.
Thanks to @Starglen and @dinakars777 for the discussion in the issue!

[algorithms-keeper]

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