Count pairs with given sum (#10282)

* added power_of_4

* deleted power_of_4

* added pairs_with_given_sum

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* Update pairs_with_given_sum.py

* Update data_structures/arrays/pairs_with_given_sum.py

---------

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

data_structures/arrays/pairs_with_given_sum.py

@@ -0,0 +1,28 @@
+#!/usr/bin/env python3
+
+"""
+Given an array of integers and an integer req_sum, find the number of pairs of array
+elements whose sum is equal to req_sum.
+
+https://practice.geeksforgeeks.org/problems/count-pairs-with-given-sum5022/0
+"""
+from itertools import combinations
+
+
+def pairs_with_sum(arr: list, req_sum: int) -> int:
+    """
+    Return the no. of pairs with sum "sum"
+    >>> pairs_with_sum([1, 5, 7, 1], 6)
+    2
+    >>> pairs_with_sum([1, 1, 1, 1, 1, 1, 1, 1], 2)
+    28
+    >>> pairs_with_sum([1, 7, 6, 2, 5, 4, 3, 1, 9, 8], 7)
+    4
+    """
+    return len([1 for a, b in combinations(arr, 2) if a + b == req_sum])
+
+
+if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod()