Problem Statement

There are k friends who want to buy N flowers. Each flower has some cost ci. But there is condition that the price of flowers changes for customer who had bought flowers before. More precisely if a customer has already bought x flowers, he should pay (x+1)*ci dollars to buy flower number i. What is the minimum amount you have to pay to buy all N flowers?

Ex Input : Output :
3 3 13
2 5 6

So first line contains n and k and next line contain ci, cost of ith flower.

Algorithm
1) Arrange all the cost in an increasing sequence…d1,d2..dn
2) Allot from end nth flower to friend 1, n-1 flower to friend 2 and so on..until k after which cost factor would be 2 and hence flower n-k would cost 2*d(n-k) to friend 1 and n-k-1 flower will cost 2*d(n-k-1) to friend 2 and so on until all flowers are bought by somebody.

If you have a different approach in mind, please share it in comments section.

Solution
 #include
 #include
 using namespace std;
int main()
 {
 int n,k;
 cin >> n >> k;
 if(k > n) k=n;
 int i,x;
 multiset s;
 multiset::iterator it;
 for(i=0;i<n;i++) { cin >> x;
 s.insert(x);
 }
//start calculating cost
 int factor = 0;
 unsigned long long int cost=0;
 int count = 0;
 it = s.end(); it--;
 while(count!=n)
 {
 if(count%k==0) factor+=1;
 cost+=(*it * factor);
 it--;
 count++;
 }
 cout << cost << endl; cin >> cost;
 }

Time Complexity
O(nlogn) to sort n cost numbers and O(n) to calculate the cost.
O(n+nlogn) => O(nlogn)
Hence time complexity will be O(nlogn)