CSES - Stick Divisions

In this problem, we're asked to find the minimum cost to divide a stick with length $x$ into $n$ sticks with given lengths. It helps to work backwards; what if we start with sticks of lengths $d_1,\ldots,d_n$ and merge them into one?

It turns out that this can be solved using Huffman Coding (also see Wikipedia). The algorithm is simple; take the two shortest sticks, merge them into one, and repeat.

If you're wondering why Huffman Coding always produces an optimal solution, see here.

Implementation

#include <iostream>
#include <queue>
using namespace std;

int main(){
		ios_base::sync_with_stdio(0); cin.tie(0);
		int x, n; cin >> x >> n;
		priority_queue<int, vector<int>, greater<int> > PQ;
		for(int i=0; i<n; i++) {
			int a; cin >> a;