#include "islands.h"
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
int n, m;
vector<int> link[100002], revLink[100002];
bool removeUselessVertices();
variant<bool, vector<int>> find_journey(int N, int M, vector<int> U, vector<int> V){
n = N, m = M;
for(int i=0; i<m; i++){
link[U[i]].push_back(V[i]);
revLink[U[i]].push_back(V[i]);
}
if(removeUselessVertices()) return false;
return true;
}
int deg[100002];
bool removed[100002];
bool removeUselessVertices(){
/// outdegree 0 삭제
queue<int> que;
for(int i=0; i<n; i++){
deg[i] = (int)link[i].size();
if(!deg[i]) que.push(i);
}
if(deg[0] == 1) for(auto y: revLink[0]) que.push(y);
while(!que.empty()){
int x = que.front(); que.pop();
removed[x] = 1;
for(auto y: revLink[x]){
deg[y]--;
if(!deg[y] && !removed[y]) que.push(y);
}
}
if(removed[0]) return true;
for(int i=0; i<n; i++){
vector<int> v;
for(auto x: link[i]) if(!removed[x]) v.push_back(x);
v.swap(link[i]);
}
return false;
}
