Cele mai apropiate puncte din plan

#include <bits/stdc++.h>
#define int long long
#define sqr(x) ((x) * (x))

using namespace std;

ifstream fin("cmap.in");
ofstream fout("cmap.out");

class point {
    public:
        int x, y;
};

int dist(const point &p1, const point &p2) {
    return sqr(p1.x - p2.x) + sqr(p1.y - p2.y);
}

int smallest_distance(const vector<point> &points, const vector<point> &points_by_y) {
    int N = points.size();
    if(N < 2)
        return 9e18L;
    vector<point> left = vector<point>(points.begin(), points.begin() + N / 2),
                  right = vector<point>(points.begin() + N / 2, points.end());
    vector<point> left_by_y, right_by_y;
    for(const point &p : points_by_y)
        if(make_pair(p.x, p.y) <= make_pair(left.back().x, left.back().y))
            left_by_y.emplace_back(p);
        else
            right_by_y.emplace_back(p);
    int d1 = smallest_distance(left, left_by_y),
        d2 = smallest_distance(right, right_by_y),
        d = min(d1, d2);
    int middle_x = left.back().x;
    vector<point> stripe;
    for(const point &p : points_by_y)
        if(sqr(p.x - middle_x) < d)
            stripe.emplace_back(p);
    int M = stripe.size();
    for(int i = 0; i < M; ++i) {
        // Lemma: Complexitatea celui de-al doilea for este O(1)
        for(int j = i + 1; j < M && sqr(stripe[j].y - stripe[i].y) < d; ++j)
            d = min(d, dist(stripe[i], stripe[j]));
    }
    return d;
}

void test_case() {
    int N;
    fin >> N;
    vector<point> points(N);
    for(point &p : points)
        fin >> p.x >> p.y;
    sort(points.begin(), points.end(), [&](const point &A, const point &B) -> bool {
        return make_pair(A.x, A.y) < make_pair(B.x, B.y);
    });
    vector<point> y_sorted = points;
    sort(y_sorted.begin(), y_sorted.end(), [&](const point &A, const point &B) -> bool {
        return A.y < B.y;
    });
    fout << fixed << setprecision(8) << sqrtl(smallest_distance(points, y_sorted)) << '\n';
}

int32_t main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    int T = 1;
    while(T--)
        test_case();
}