tot_pair_div_by_k.cpp

#include <iostream>
using namespace std;

int nChoosek( unsigned n, unsigned k )
{
    if (k > n) 
		return 0;
    if (k * 2 > n) 
		k = n-k;
    if (k == 0) 
		return 1;
 
    int result = n, temp;
    for( int i = 2; i <= k; ++i ) {
        result *= (n-i+1);
        result /= i;
    }
    return result;
}

int main() 
{
    int t;
    cin>>t;
    while(t--)
    {
        int n ,k;
        cin>>n >>k;
        int arr [k];
        int r = n/k;
        int rem = n%k;
        for(int i = 0; i< k; i++)
        {
            arr[i] = r;
        }
        for(int i = 1; i <= rem; i++)
        {
            arr[i]++;
        }
        int res = 0;
        res += nChoosek(arr[0],2);
        if(k%2 == 0)
        {
            int i;
            for(i =1; i<(k+1)/2; i++)
            {
                res += ( nChoosek(arr[i],1) * nChoosek(arr[k-i],1) );
            }
            res += nChoosek(arr[i],2);
        }
        else
        {
            for(int i=1; i< (k+1)/2; i++)
            {
                res += ( nChoosek(arr[i],1) * nChoosek(arr[k-i],1) );
            }
        }
        cout<<res<<"\n";
    }   
    return 0;
}