Line Intersections

// ~/.vim/cpp_template.cpp
#include <bits/stdc++.h>
#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <limits>

#define REP(i,a,b) for(int i = a; i < b; i++)

// Type Aliases for 1D and 2D vectors with initialization
#define vi(n, val) vector<int>(n, val) // 1D vector of ints with size n, initialized to val
#define vll(n, val) vector<long long>(n, val) // 1D vector of long longs with size n, initialized to val
#define ll long long
#define vvi(n, m, val) vector<vector<int>>(n, vector<int>(m, val)) // 2D vector of ints(n x m), initialized to val
#define vvll(n, m, val) vector<vector<long long>>(n, vector<long long>(m, val)) // 2D vector of long longs(n x m), initialized to val

#define LL_INF std::numeric_limits<long long int>::max()
#define INF std::numeric_limits<int>::max()


using namespace std;


template <typename T>
void pV(const std::vector<T>& vec, const std::string& label = "Vector") {
    std::cout << label << ": [ ";
    for(const auto& elem : vec) {
        std::cout << elem << " ";
    }
    std::cout << "]" << std::endl;
}


void dfs(int s, vector<bool> *visited, vector<int>(*adj)[]) {
    if((*visited)[s]) return;
   (*visited)[s] = true;
    // process node s


    for(auto u:(*adj)[s]) {
        dfs(u, visited, adj);
    }
}
/*
// DEPTH-FRIRST-SEARCH
vector<int> adj[N];                                                          
vector<bool> visited(N, false);
int u, v;
for(int i = 0; i < M;i++){
    cin >> u >> v;
    u--;
    v--;
    adj[u].push_back(v);
    adj[v].push_back(u);
}
*/

vector<long long> dijkstra(int N, int x, vector<vector<pair<int,int>>> *adj){
    vector<bool> processed(N, false);
    vector<long long> distance(N, LL_INF);
    priority_queue<pair<long long,int>> q;
    //for(int i = 1; i <= n; i++) distance[i] = INF;
    distance[x] = 0;
    q.push({0,x});
    while(!q.empty()) {
        int a = q.top().second; q.pop();
        if(processed[a]) continue;
        processed[a] = true;
        for(auto u :(*adj)[a]) {
            int b = u.first, w = u.second;
            if(distance[a]+w < distance[b]) {
                distance[b] = distance[a]+w;
                q.push({-distance[b],b});
            }
        }
    }
    return distance;


}


/*
DIJKSTRA
//vector<vector<pair<int,int>>> adj(N, vector<pair<int, int>>(N));
*/

const int HORSTART = 0;
const int HOREND = 1;
const int VERTICAL = 2;

struct Event {
    int x, y1, y2, type;
    Event(int x_, int y1_, int y2_, int type_) : x(x_), y1(y1_), y2(y2_), type(type_) {}
};

vector<int> cpr_coords(vector<int>& coords) {
    vector<int> sorted_coords = coords;
    sort(sorted_coords.begin(), sorted_coords.end());
    sorted_coords.erase(unique(sorted_coords.begin(), sorted_coords.end()), sorted_coords.end());

    vector<int> compressed(coords.size());
    for(size_t i = 0; i < coords.size(); ++i) {
        compressed[i] = lower_bound(sorted_coords.begin(), sorted_coords.end(), coords[i]) - sorted_coords.begin();
    }
    return compressed;
}

void update_BIT(vector<int>& BIT, int index, int value) {
    while(index <(int)BIT.size()) {
        BIT[index] += value;
        index += index & -index;
    }
}

int query_BIT(const vector<int>& BIT, int index) {
    int sum = 0;
    while(index > 0) {
        sum += BIT[index];
        index -= index & -index;
    }
    return sum;
}

int cnt_ints(const vector<Event>& events, const vector<int>& all_y_coords) {
    vector<int> compressed_y = cpr_coords(const_cast<vector<int>&>(all_y_coords));

    map<int, int> y_to_compressed;
    for(size_t i = 0; i < all_y_coords.size(); ++i) {
        y_to_compressed[all_y_coords[i]] = compressed_y[i] + 1;
    }

    int max_y_index = *max_element(compressed_y.begin(), compressed_y.end()) + 1;
    vector<int> BIT(max_y_index + 1, 0);

    int intersections = 0;

    for(const Event& event : events) {
        if(event.type == HORSTART) {
            update_BIT(BIT, y_to_compressed[event.y1], 1);
        }
        else if(event.type == HOREND) {
            update_BIT(BIT, y_to_compressed[event.y1], -1);
        }
        else if(event.type == VERTICAL) {
            int y1_compressed = y_to_compressed[event.y1];
            int y2_compressed = y_to_compressed[event.y2];
            intersections += query_BIT(BIT, y2_compressed) - query_BIT(BIT, y1_compressed - 1);
        }
    }

    return intersections;
}

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    // Your code starts here
    int n;
    cin >> n;

    vector<Event> events;
    vector<int> all_y_coords;

    REP(i, 0, n){
        int x1, y1, x2, y2;
        cin >> x1 >> y1 >> x2 >> y2;

        if(y1 == y2) {
            if(x1 > x2) swap(x1, x2);
            events.emplace_back(x1, y1, y1, HORSTART);
            events.emplace_back(x2, y1, y1, HOREND);
            all_y_coords.push_back(y1);
        } else if(x1 == x2) {
            if(y1 > y2) swap(y1, y2);
            events.emplace_back(x1, y1, y2, VERTICAL);
            all_y_coords.push_back(y1);
            all_y_coords.push_back(y2);
        }
    }

    sort(events.begin(), events.end(), [](const Event& a, const Event& b) {
        if(a.x != b.x) return a.x < b.x;
        return a.type < b.type;
    });

    int result = cnt_ints(events, all_y_coords);
    cout << result << endl;

    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: WRONG ANSWER

Test 7

Verdict: ACCEPTED

Test 8

Verdict: WRONG ANSWER

Test 9

Verdict: ACCEPTED

Test 10

Verdict: WRONG ANSWER

Test 11

Verdict: WRONG ANSWER

Test 12

Verdict: WRONG ANSWER

Test 13

Verdict: WRONG ANSWER

Test 14

Verdict: ACCEPTED

Test 15

Verdict: WRONG ANSWER

Test 16

Verdict: WRONG ANSWER

Test 17

Verdict: WRONG ANSWER

Test 18

Verdict: WRONG ANSWER

Test 19

Verdict: WRONG ANSWER

Test 20

Verdict: WRONG ANSWER

Test 21

Verdict: WRONG ANSWER

Test 22

Verdict: WRONG ANSWER

Test 23

Verdict: WRONG ANSWER

Test 24

Verdict: WRONG ANSWER

Test 25

Verdict: WRONG ANSWER

Test 26

Verdict: WRONG ANSWER

Test 27

Verdict: WRONG ANSWER

Test 28

Verdict: WRONG ANSWER

Test 29

Verdict: WRONG ANSWER

Test 30

Verdict: WRONG ANSWER

Test 31

Verdict: WRONG ANSWER

Test 32

Verdict: WRONG ANSWER

Test 33

Verdict: WRONG ANSWER

Test 34

Verdict: WRONG ANSWER

Test 35

Verdict: WRONG ANSWER

Test 36

Verdict: WRONG ANSWER

Test 37

Verdict: WRONG ANSWER

Test 38

Verdict: WRONG ANSWER

Test 39

Verdict: WRONG ANSWER

Test 40

Verdict: WRONG ANSWER