#include "islands.h"
#include<bits/stdc++.h>
using namespace std;
vector<int> u, v;
vector<vector<int>> g;
vector<int> seen, es;
vector<int> p, t, c;
void dfs(int cur){
if (!c.empty()) return;
seen[cur] = 1;
for (int next : g[cur]){
if (!c.empty()) return;
if (es[next]) continue;
es[next] = true;
int nn = u[next]+v[next]-cur;
if (seen[nn] == 2) continue;
if (seen[nn] == 1){
if (nn == v[next]) c.push_back(next);
else c.push_back(next+1);
while (cur != nn){
if (cur == v[p[cur]]) c.push_back(p[cur]);
else c.push_back(p[cur]+1);
cur = u[p[cur]]+v[p[cur]]-cur;
}
while (cur != 0){
if (cur == v[p[cur]]) t.push_back(p[cur]);
else t.push_back(p[cur]);
cur = u[p[cur]]+v[p[cur]]-cur;
}
reverse(c.begin(), c.end());
reverse(t.begin(), t.end());
return;
}
if (seen[v[next]] == 0){
p[v[next]] = next;
dfs(v[next]);
}
}
seen[cur] = 2;
}
variant<bool, vector<int>> find_journey(int N, int M, vector<int> U, vector<int> V){
u = U;
v = V;
g.resize(N);
for (int i=0; i<M; i+=2){
g[U[i]].push_back(i);
g[V[i]].push_back(i);
}
seen.resize(N, 0);
es.resize(M, 0);
p.resize(N);
dfs(0);
if (c.empty()) return false;
vector<int> res;
for (int x : t) res.push_back(x);
for (int x : c) res.push_back(x);
reverse(c.begin(), c.end());
for (int x : c) res.push_back(x^1);
for (int x : c) res.push_back(x);
reverse(c.begin(), c.end());
for (int x : c) res.push_back(x^1);
reverse(t.begin(), t.end());
for (int x : t) res.push_back(x);
return res;
}
