Count Pairs with the Given Sum

In this article, we will solve the problem of counting the number of pairs in an array that sum up to a given target value.

Problem Statement: Given an array of integers and a target sum, write a function to count the number of pairs of integers in the array whose sum is equal to the target sum.

Example:

Input: arr = [1, 5, 7, 4, 2], target_sum = 6

Output: 2

Explanation: The pairs with sum 6 are (1, 5) and (4, 2).

Solution:

To solve this problem we can use a hash map to store the frequencies of elements we have seen so far. We will iterate through the array and for each element, check if the complement (target sum - current element) exists in the hash map. If it does, we add the frequency of the complement to our count of pairs.

Steps to Solve:

1.    Initialize a hash map to store frequencies of elements.

2.    Initialize a variable to count the pairs.

3.    Iterate through the array:

o   For each element, calculate its complement with respect to the target sum.

o   If the complement exists in the hash map, add its frequency to the count of pairs.

o   Update the frequency of the current element in the hash map.

4.    Return the count of pairs.

Time Complexity: O(N)

Space Complexity: O(N)