ICPC Camp 6 Problem B

#include <bits/stdc++.h>
using namespace std;

mt19937_64 RNG(chrono::steady_clock::now().time_since_epoch().count());
#define endl "\n"
#define int long long
#define INF (int)2e18

#ifdef LOCAL
    #include "debug.h"
#else
    #define debug(...)
    #define _start_
    #define _debug_
    #define _end_
#endif

void solve(){
    int n,m;
    cin>>n>>m;
    vector<pair<int, int>> edges;
    for(int i=0 ; i<m ; i++){
        int a, b;
        cin>>a>>b;
        edges.push_back({a, b});
    }
    set<pair<int, int>> edgest;
    vector<pair<int, int>> doubleedges;
    vector<int> indegree(n+1, 0);
    for(int i=0 ; i<m ; i++){
        int a = edges[i].first;
        int b = edges[i].second;
        indegree[b]++;
        if(edgest.find({b, a}) != edgest.end()){
            doubleedges.push_back({a, b});
            indegree[b]--;
            indegree[a]--;
        }
        edgest.insert({a, b});
    }
    debug(doubleedges)
    int ct = 0;
    for(int i=1 ; i<=n ; i++){
        if(indegree[i] > 0){
            ct++;
        }
    }
    debug(indegree)
    if(ct == n){
        cout<<"YES"<<endl;
        return;
    }
    vector<int> done(n+1, 0);
    for(int i=1 ; i<=n ; i++){
        if(indegree[i] > 0){
            done[i] = 1;
        }
    }
    vector<vector<int>> graph(n+1, vector<int>());
    for(auto edge : doubleedges){
        int u = edge.first;
        int v = edge.second;
        graph[u].push_back(v);
        graph[v].push_back(u);
    }
    vector<vector<int>> grpele;
    vector<int> color(n+1, 0);
    for(int i=1 ; i<=n ; i++){
        bool isCycle = false;
        vector<int> elements;
        function<void(int, int)> dfs = [&](int node, int parent){
            color[node] = 1;
            for(auto j : graph[node]){
                if(j == parent){
                    continue;
                }
                if(color[j] == 0){
                    dfs(j, node);
                }
                else if(color[j] == 1){
                    isCycle = true;
                }
            }
            color[node] = 2;
            elements.push_back(node);
        };
        if(color[i] == 0){
            dfs(i, -1);
            if(isCycle){
                for(auto ele : elements){
                    done[i] = 1;
                }
            }
            grpele.push_back(elements);
        }
    }
    for(auto elements : grpele){
        int check = false;
        for(auto ele : elements){
            if(indegree[ele] > 0){
                check = true;
            }
        }
        if(check){
            for(auto ele : elements){
                done[ele] = 1;
            }
        }
    }
    ct = 0;
    for(int i=1 ; i<=n ; i++){
        if(done[i]){
            ct++;
        }
    }
    if(ct == n){
        cout<<"YES"<<endl;
        return;
    }
    cout<<"NO"<<endl;
    return;
}

signed main(){
    _start_
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    _debug_
    int _t = 1;
    // cin >> _t;
    for(int  _ = 1 ;  _ <= _t ; _++){
        solve();
    }
    _end_
}