Envío 2519

Código

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

vector<int> seen;
vector<int> topo;
void dfs(int from, vector<vector<int>> &G){
    seen[from] = 1;
    for(auto adj:G[from])
        if(!seen[adj])
            dfs(adj,G);
}

void topoSort(int from,vector<vector<int>> &G){
    seen[from] = 1;
    for(auto adj:G[from])
        if(!seen[adj])
            topoSort(adj,G);
    topo.push_back(from);
}

int main(){
    int n,m;
    scanf(" %d %d",&n,&m);
    seen.resize(n,0);
    vector<vector<int>> G(n),GI(n);
    int already = 0;
    while(m--){
        int a,b;
        scanf(" %d %d",&a,&b);
        GI[b].push_back(a);
        G[a].push_back(b);
        already = already | (a == b);
    }
    if(already){
        puts("Yes");
        return 0;
    }
    for(int i = 0; i < n;++i)
        if(!seen[i])
            topoSort(i,GI);
    reverse(topo.begin(),topo.end());
    seen.clear();
    seen.resize(n,0);
    int components = 0;
    for(auto va:topo)
        if(!seen[va]){
            dfs(va,G);
            ++components;
        }
    if(components == n)
        puts("No");
    else puts("Yes");
    return 0;
}