#include "islands.h"
#include<bits/stdc++.h>
using namespace std;
variant<bool, vector<int>> find_journey(int N, int M, vector<int> U, vector<int> V){
vector<vector<int>> g(N);
for (int i=0; i<M; i+=2){
g[U[i]].push_back(i);
g[V[i]].push_back(i+1);
}
vector<int> p1, p2;
int cur = 0, prev = 0;
while (true){
if (g[cur].size() == 0 || (g[cur].size() == 1 && cur != 0)) return false;
if (g[cur].size() == 3 || (g[cur].size() == 2 && cur == 0)){
break;
}
int next = U[g[cur][0]]+V[g[cur][0]]-cur;
if (next == prev){
next = U[g[cur][1]]+V[g[cur][1]]-cur;
p1.push_back(g[cur][1]+(next == U[g[cur][1]]));
}
else p1.push_back(g[cur][0]+(next == U[g[cur][0]]));
prev = cur;
cur = next;
}
vector<int> res;
for (int x : p1) res.push_back(x);
for (int x : p2) res.push_back(x);
reverse(p1.begin(), p1.end());
for (int x : p1) res.push_back(x);
return res;
}
