HackerRank "Minimum Average Waiting Time" !

#include <iostream>
#include <fstream>
#include <iterator>
#include <vector>
#include <stack>
#include <cstring>
#include <climits>
#include <algorithm>
#include <unordered_map>
#include <unordered_set>
#include <set>
#include <queue>
using namespace std;

struct Rec
{
Rec(int s, int d) : start(s), duration(d){}
int start;
int duration;

bool operator < (const Rec& p) const {
return start < p.start;
}
};

struct Cmp
{
bool operator()(const Rec& p1, const Rec& p2) {
return p1.duration > p2.duration;
}
};

int main()
{

int n; cin >> n;

//    Get input and sort by arriving time
vector<Rec> in;
for(int i = 0; i < n; i ++)
{
int s, t; cin >> s >> t;
in.push_back(Rec(s, t));
}
sort(in.begin(), in.end());

//    Shortest Job First algorithm
long long ans = 0, end = 0;
priority_queue<Rec, vector<Rec>, Cmp> q;

int i = 0;
while ( i < n || !q.empty())
{
if (q.empty()) // some gap with NO customers
{
end = max(end, (long long)(in[i].start));
}
//    add all arrived customers
while(i < n && in[i].start <= end)
{
q.push(in[i]);
i ++;
}
Rec r = q.top();
end += r.duration;
ans += end - r.start;
q.pop();
}
cout << ans / n << endl;
return 0;
}