A. Мишка и условие дружбы

main.cc

#include <bits/stdc++.h>

using namespace std;

using ll = long long;

int main(int argc, char **argv) {
    ios::sync_with_stdio(false);
    srand(time(0));

#ifndef ONLINE_JUDGE
    freopen("/home/acarus/input.txt", "r", stdin);
#endif

    int n, m;
    cin >> n >> m;

    vector<set<int>> g(n + 1, set<int>());

    int u, v;
    for (int i = 0; i < m; ++i) {
        cin >> u >> v;
        g[u].insert(v);
        g[v].insert(u);
    }

    for (int i = 1; i <= n; ++i) {
        vector<pair<int, int>> pp;
        for (int to1: g[i]) {
            for (int to2: g[i]) {
                if (to1 == to2) continue;
                if (!g[to1].count(to2) || !g[to2].count(to1)) {
                    cout << "NO" << endl;
                    return 0;
                }
                pp.push_back(make_pair(to1, to2));
            }
        }

        for (auto& p: pp) {
            g[p.first].erase(p.second);
            g[p.second].erase(p.first);
        }
    }
    cout << "YES" << endl;
    return 0;
}