#include "islands.h"
#include <algorithm>
#include <array>
#include <cassert>
#include <iostream>
#include <iterator>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <string>
#include <tuple>
#include <utility>
#include <variant>
#include <vector>
using std::array;
using std::pair;
using std::string;
using std::tuple;
using std::vector;
using ll = long long;
constexpr int inf = (1 << 30) - 1;
constexpr ll infll = (1ll << 62) - 1;
template <class F> struct make_fixed : private F {
explicit make_fixed(F&& f) : F(std::forward<F>(f)) {}
template <class... Args> decltype(auto) operator()(Args&&... args) const {
return F::operator()(*this, std::forward<Args>(args)...);
}
};
template <class T, class... Args> void DBG(const T& x, const Args&... args) {
std::cerr << x;
if constexpr (sizeof...(args)) {
std::cerr << ' ';
DBG(args...);
} else {
std::cerr << std::endl;
}
}
template <class T> void DBG(const vector<T>& v) {
std::cerr << '[';
bool f = false;
for (const T& x : v) {
if (f) std::cerr << ", ";
f = true;
std::cerr << x;
}
std::cerr << ']' << std::endl;
}
std::variant<bool, vector<int>> find_journey(int N, int M, vector<int> U, vector<int> V) {
vector<vector<int>> G(N), rG(N);
for (int i = 0; i < M; ++i) {
G[U[i]].push_back(V[i]);
rG[V[i]].push_back(U[i]);
}
vector<char> dead(N);
vector<int> out(N);
const auto kill = make_fixed([&](auto&& kill, int u) -> void {
if (dead[u]) return;
dead[u] = true;
for (const int v : rG[u]) {
if ((--out[v]) == 0) kill(v);
}
});
for (int i = 0; i < N; ++i) {
out[i] = (int)G[i].size();
if (out[i] == 0) kill(i);
}
for (int u = 0, w;; u = w) {
w = -1;
for (const int v : G[u]) {
if (!dead[v]) {
if (w == -1) w = v;
else return true;
}
}
if (w == -1) return false;
dead[u] = true;
u = w;
}
}
