SMMR-169: Thailand 4.0

#include <bits/stdc++.h>

using namespace std;

using lli = long long;
using pL = pair <lli, lli>;
using pLL = pair <pL, pL>;

int main(){

    int n;
    scanf("%d", &n);

    vector <pLL> event;

    for(int i=1;i<=n;i++){
        lli x1, x2, y1, y2;
        scanf("%lld%lld%lld%lld", &x1, &x2, &y1, &y2);
        event.push_back({{x1, 2}, {y1, y2}});
        event.push_back({{x2, 1}, {y1, y2}});
    }

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

    multiset <pL> H;
    lli pre_x = -1, ans = 0;
    for(auto e: event){

        lli x, ev, y1, y2;
        x = e.first.first;
        ev = e.first.second;
        y1 = e.second.first;
        y2 = e.second.second;

        if(pre_x == -1){
            pre_x = x;
            H.insert({y2, 1}); // 1 delete
            H.insert({y1, 2}); // 2 insert
            continue;
        }

        // calculate area
            // find x
            lli sum_x = x - pre_x;
            // find y
            lli sum_y = 0, pre_y = -1, cnt = 0;
            for(auto h: H){
                if(h.second == 2) {
                    if(pre_y == -1) pre_y = h.first;
                    cnt ++;
                }
                else{
                    cnt --;
                    if(cnt == 0){
                        sum_y += h.first - pre_y;
                        pre_y = -1;
                    }
                }
            }
            // area = x * y
            ans += (lli) sum_x * sum_y;

        // if ev = 1, delete H // ev = 2, insert H
        if(ev == 1){
            H.erase(H.find({y2, 1})); // 1 delete
            H.erase(H.find({y1, 2})); // 2 insert
        }
        else if(ev == 2){
            H.insert({y2, 1}); // 1 delete
            H.insert({y1, 2}); // 2 insert
        }

        // set variable
        pre_x = x;
    }

    printf("%lld", ans);

    return 0;
}