AC了但是不懂

#include<bits/stdc++.h>
#define N 300010
#define int long long
using namespace std;
int ans,n,p[N];
priority_queue<int,vector<int>,greater<int> >q;
signed main(){
	ios::sync_with_stdio(0);
	cin.tie(0);cout.tie(0);
	cin>>n;
	for(int i = 1;i<=n;i++) 
		cin>>p[i];
	for(int i = 1;i<=n;i++){
		q.push(p[i]);
		if(q.size()&&q.top()<p[i]){
			ans+=p[i]-q.top();
			q.pop();
			q.push(p[i]);
		}
	}
	cout<<ans;
	return 0;
}