count_pair_sum.cpp

// { Driver Code Starts
//Initial template for C++

#include <bits/stdc++.h>
using namespace std;

// } Driver Code Ends
//User function template for C++

class Solution
{
public:
    int getPairsCount(int arr[], int n, int sum)
    {
        // code here
        unordered_map<int, int> m;
        // Store counts of all elements in map m
        for (int i = 0; i < n; i++)
            m[arr[i]]++;

        int twice_count = 0;

        // iterate through each element and increment the
        // count (Notice that every pair is counted twice)
        for (int i = 0; i < n; i++)
        {
            twice_count += m[sum - arr[i]];

            // if (arr[i], arr[i]) pair satisfies the condition,
            // then we need to ensure that the count is
            // decreased by one such that the (arr[i], arr[i])
            // pair is not considered
            if (sum - arr[i] == arr[i])
                twice_count--;
        }

        // return the half of twice_count
        return twice_count / 2;
    }
};

// { Driver Code Starts.

int main()
{
    int t;
    cin >> t;
    while (t--)
    {
        int n, k;
        cin >> n >> k;
        int arr[n];
        for (int i = 0; i < n; i++)
        {
            cin >> arr[i];
        }
        Solution ob;
        auto ans = ob.getPairsCount(arr, n, k);
        cout << ans << "\n";
    }

    return 0;
} // } Driver Code Ends