## Solución al problema número 102501K de Codeforces - 102501K.

Cualquier duda no dudes en contactar.

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

int N,E;
const int MAXN = 100100;
struct edge{
int from, to;
edge(){}
edge(int a, int b){
from = a;
to = b;
}
};

vector<edge> graph[MAXN];
vector<edge> rgraph[MAXN];

int main(){
int CHECK;
scanf("%d %d %d",&N,&E,&CHECK);
bool visited[N+2],visited2[N+2];
int visited1[N+2];
memset(visited,false,sizeof(visited));
set<int> C,S;
for(int i=0;i<E;i++){
int from, to; scanf("%d %d",&from, &to);
if(to==CHECK && from!=CHECK) { C.insert(from); S.insert(from); visited[from]=true;} //nodes with direct path and diffent to check
else if(to!=CHECK && from!=CHECK) { graph[from].push_back(edge(from,to));  rgraph[to].push_back(edge(to,from));}//delete edge
}
map<int,bool> M;
memset(visited1,-1,sizeof(visited1));
for(int i:C){
if(M[i]) continue;
memset(visited2,false,sizeof(visited2));
queue<int> q; q.push(i);
bool flag = true;
while(!q.empty() && flag){
int ele = q.front();
q.pop();
for(unsigned int j=0;j<graph[ele].size();j++){
if(visited[graph[ele][j].to] && graph[ele][j].to!=i) {
S.erase(i);
M[i]=true;
if(visited1[graph[ele][j].from]==-1){
queue<int> test;
test.push(graph[ele][j].from); visited1[i]=i;
while(!test.empty()){
int aux = test.front();
test.pop();
for(unsigned int l=0;l<rgraph[aux].size();l++){
if(rgraph[aux][l].to!=i && visited1[rgraph[aux][l].to]==-1 && visited2[rgraph[aux][l].to]) test.push(rgraph[aux][l].to);
if(visited2[rgraph[aux][l].to]) {
visited1[rgraph[aux][l].to]=graph[ele][j].to;
}
}
}
}
flag=false;
break;
}
if(visited1[graph[ele][j].to]!=-1 && visited1[graph[ele][j].to]!=i){
//cout<<"PODA\n";
S.erase(i);
M[i]=true;
flag=false;
break;
}
if(!visited2[graph[ele][j].to]) q.push(graph[ele][j].to);
visited2[graph[ele][j].to]=true;
}
}
}
printf("%d\n",S.size());
for(auto s:S){
printf("%d\n",s);
}
return 0;
}

/*
5 7 0
1 0
2 1
1 3
3 2
2 0
3 4
4 0
*/

/*
5 6 0
2 0
1 0
1 6
3 6
6 2
2 3
*/
``````

### No te pierdas nada.

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