#include "islands.h"
#include <variant>
#include<iostream>
#include <vector>
using namespace std;
using pii = pair<int, int>;
#define forn(i, n) for(int i=0; i<(int)n; ++i)
#define PB push_back
const int MAXN = 100010;
int n, m;
bool marked[MAXN], inst[MAXN], vis[MAXN];
vector<int> g[MAXN];
vector<int> st;
bool dfs1(int v){
vis[v]=inst[v]=true;
st.PB(v);
bool ret=false;
for(int to:g[v]) if(!marked[to] && !vis[to]){
ret|=dfs1(to);
}
else if(inst[to]){
ret=true;
}
st.pop_back();
inst[v]=false;
return ret;
}
variant<bool, vector<int>> find_journey(int N, int M, vector<int> U, vector<int> V) {
n=N, m=M;
forn(i, m) g[U[i]].PB(V[i]);
int cur=0;
while(true){
vector<int> nxt;
for(int to:g[cur]) if(!marked[to]){
if(dfs1(to)) nxt.PB(to);
forn(i, n) vis[i]=false;
}
if(nxt.empty()) return false;
if((int)nxt.size() > 1){
vector<int> emp;
return emp;
}
marked[cur]=true;
cur=nxt.back();
}
}
