added pairs_with_given_sum

data_structures/arrays/pairs_with_given_sum.py

@@ -0,0 +1,48 @@
+"""
+Author  : Siddharth Warrier
+Date    : October 3, 2023
+
+Task:
+Count the no of pairs in a given array with given sum
+
+Implementation notes: Using hashing
+The idea is that we hash the array in a dictionary
+Then go throught the elemnts 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, k):
+    """
+    Return the no. of pairs with sum k
+
+    >>> 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 = {}
+    for i in arr:
+        if i in d:
+            d[i]+=1
+        else:
+            d[i] = 1
+    ans = 0
+    for i in arr:
+        d[i]-=1
+        if k-i in d and d[k-i]!=0:
+            ans+=d[k-i]
+        d[i]+=1
+    return ans//2
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()