ДолойСписывание

#include <iostream>
#include <fstream>
#include <map>
#include <set>
#include <deque>
#include <vector>
using namespace std;

bool bfs(int start, map<int, set<int>> adjSets, vector<int> used) {

    deque<int> points;
    points.push_back(start);
    used[start] = 1;

    while(!points.empty()) {
        int current = points.front();

        set<int> adjSet = adjSets[current];
        for (auto neighbour: adjSet) {
            if (used[neighbour] == 0) {
                points.push_back(neighbour);
                used[neighbour] = 1;
            } else {
                return false;
            }
        }

        points.pop_front();
        used[current] = 2;
    }

    return true;
}

int main() {
    ifstream file("input.txt");
    int n, m;
    file >> n >> m;

    map<int, set<int>> adjSets;
    vector<int> used;
    used.resize(n, 0);

    for (int i = 0; i < m; i++) {
        int from, to;
        file >> from;
        file >> to;
        adjSets[from-1].insert(to-1);
    }

    for (int i = 0; i < n; i++) {
        if (!bfs(i, adjSets, used)) {
            cout << "NO" << endl;
            return 0;
        }
    }


    cout << "YES" << endl;
    return 0;
}