Count pairs with given sum (#10282)

* added power_of_4

* deleted power_of_4

* added pairs_with_given_sum

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* updated the comment

* updated return hint

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* updated code

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* added the problem link and used defaultdict

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* corrected import formatting

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* Update pairs_with_given_sum.py

* Update data_structures/arrays/pairs_with_given_sum.py

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Christian Clauss <[email protected]>