Count pairs with given sum

data_structures/arrays/pairs_with_given_sum.py

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+"""
+Author  : Siddharth Warrier
+Date    : October 3, 2023
+
+Task:
+Count the no of pairs in a given array with given sum
+Problem URL- https://practice.geeksforgeeks.org/problems/count-pairs-with-given-sum5022/0
+
+Implementation notes: Using hashing
+The idea is that we hash the array in a dictionary
+Then go through the elements of the array
+We subtract this with the given sum
+and check if that is there in the array
+We also check the edge cases like if there are multiple same elements
+Finally we divide the count by 2
+to avoid the same pair getting counted twice
+"""
+
+
+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
+    """
+    d = defaultdict(int)
+    for i in arr:
+        d[i] += 1
+    ans = 0
+    for i in arr:
+        d[i] -= 1
+        if req_sum - i in d and d[req_sum - i] != 0:
+            ans += d[req_sum - i] - 1
+        d[i] += 1
+    return ans // 2
+
+
+if __name__ == "__main__":
+    import doctest
+    from collections import defaultdict
+
+    doctest.testmod()