315A - Sereja and Bottles - CodeForces Solution

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <algorithm>
#include <cmath>
#include <vector>
#include <set>
#include <map>
#include <stack>
#include <unordered_set>
#include <unordered_map>
#include <queue>
#include <ctime>
#include <cassert>
#include <complex>
#include <string>
#include <cstring>
#include <chrono>
#include <random>
#include <bitset>
#include <array>
#include <any>

using namespace std;

#ifdef LOCAL
#define eprintf(...)                  \
    {                                 \
        fprintf(stderr, __VA_ARGS__); \
        fflush(stderr);               \
    }
#else
#define eprintf(...) 42
#endif

using ll = long long;
using ld = long double;
using uint = unsigned int;
using ull = unsigned long long;
template <typename T>
using pair2 = pair<T, T>;
using pii = pair<int, int>;
using pli = pair<ll, int>;
using pll = pair<ll, ll>;
int MOD = 1e9 + 7;
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
ll myRand(ll B)
{
    return (ull)rng() % B;
}


#define pb push_back
#define mp make_pair
#define all(x) (x).begin(), (x).end()
#define fi first
#define se second
#define print(...) cout << ... << endl;
#define nl "\n"
#define REPL(x, n) for(int x = 0; x < n; ++x)


clock_t startTime;
double getCurrentTime() {
    return (double)(clock() - startTime) / CLOCKS_PER_SEC;
}

void printiv(vector<ll> v) {
    for (int ele : v) {
        cout << ele << " ";
    }
    cout << endl;
}

pair<int,int> arr[1001] {mp(-1, -1)};

int main() {
    ios::sync_with_stdio(0);
	cin.tie(0);

    int n;
    int a, b;

    cin >> n;
    unordered_set<int> needstobeopened = {};

    REPL(i, n) {
        cin >> a >> b;
        arr[i] = mp(a, b);
        needstobeopened.insert(i);
    }

    unordered_set<int> hasbeenopened = {};

    stack<int> isopenedoniter = {};
    int cnt = 0;
    REPL(i, n) {
        b = arr[i].second;
        REPL(j, n) {
            if(i == j) continue;
            if (arr[j].first == b && (hasbeenopened.count(b) == 0 || needstobeopened.count(j) > 0)) {
                //cout << b << nl;
                cnt += 1;
                needstobeopened.erase(j);
                isopenedoniter.push(b);
            }
        }
        while(!isopenedoniter.empty()) {
            int tmp;
            tmp = isopenedoniter.top();
            isopenedoniter.pop();
            hasbeenopened.insert(tmp);
        }
    }

    cout << n - cnt  << nl;

    return 0;
}