Closest points

#include <bits/stdc++.h>

using namespace std;

#define debug(...) Debug(#__VA_ARGS__, __VA_ARGS__);
template <typename... Args>
void Debug(const char* names, Args&&... args) {
   std::cerr << names << " = ";
   ((std::cerr << args << ", "), ...) << "\b\b " << std::endl;
}
#define ll long long
#define ull unsigned long long
#define vi vector<int>

ull distance2(const pair<ll,ll>& p1, const pair<ll,ll>& p2) {
    ll x1 = p1.first, y1 = p1.second;
    ll x2 = p2.first, y2 = p2.second;
    ull dx = (ull) abs(x1 - x2);
    ull dy = (ull) abs(y1 - y2);
    return dx*dx + dy*dy;
}

int main() {
    int n;
    cin >> n;

    vector<pair<ll,ll>> points (n);
    for (int i = 0; i < n; i++)
    {
        cin >> points[i].first >> points[i].second;
    }

    sort(points.begin(), points.end());

    ull min_d2 = ULLONG_MAX;
    for (int i = 0; i < n; i++)
    {
        int iback = i-1;
        debug(i)
        while (iback >= 0) {
            ull d2 = distance2(points[i], points[iback]);
            ull dx2 = (points[i].first - points[iback].first)*(points[i].first - points[iback].first);
            if (d2 < min_d2) min_d2 = d2;
            if (dx2 > min_d2) break;
            iback--;
        }
    }
    
    cout << min_d2 << endl;
    

    return 0;
}

Test 1

Verdict: ACCEPTED

i = 0,  
i = 1,  
i = 2,  
i = 3,  
i = 4,  
i = 5,  
i = 6,  
i = 7,  
i...

Test 2

Verdict: TIME LIMIT EXCEEDED

Test 3

Verdict: TIME LIMIT EXCEEDED

Test 4

Verdict: TIME LIMIT EXCEEDED

Test 5

Verdict: ACCEPTED

i = 0,  
i = 1,  
i = 2,  
i = 3, 

Test 6

Verdict: TIME LIMIT EXCEEDED

Test 7

Verdict: ACCEPTED

i = 0,  
i = 1,  
i = 2,  
i = 3, 

Test 8

Verdict: ACCEPTED

i = 0,  
i = 1,  
i = 2,  
i = 3, 

Test 9

Verdict: ACCEPTED

i = 0,  
i = 1, 

Test 10

Verdict: TIME LIMIT EXCEEDED

Test 11

Verdict: ACCEPTED

i = 0,  
i = 1,  
i = 2,  
i = 3,  
i = 4,  
i = 5,  
i = 6,  
i = 7, 

Test 12

Verdict: ACCEPTED

i = 0,  
i = 1,  
i = 2, 

Test 13

Verdict: TIME LIMIT EXCEEDED

Test 14

Verdict: ACCEPTED

i = 0,  
i = 1,  
i = 2,  
i = 3, 

Test 15

Verdict: ACCEPTED

i = 0,  
i = 1,  
i = 2,  
i = 3,  
i = 4,  
i = 5,  
i = 6,  
i = 7,  
i...

Test 16

Verdict: TIME LIMIT EXCEEDED

Test 17

Verdict: TIME LIMIT EXCEEDED

Test 18

Verdict: ACCEPTED

i = 0,  
i = 1,  
i = 2,