#include "islands.h"
#include <variant>
#include <vector>
#include <queue>
#include <iostream>
using namespace std;
const int maxn = 1e5 + 10;
int seen[maxn], in_deg[maxn], out_deg[maxn];
int n, m;
vector < int > links[maxn], rev_links[maxn];
vector < pair < int, int > > graph[maxn];
vector < int > path[3];
int clear_useless()
{
queue < int > q;
for (int i = 0; i < n; i ++)
{
if (!out_deg[i])
q.push(i);
}
int start = 0;
while(!q.empty() || out_deg[start] <= 1)
{
if (!q.empty())
{
int node = q.front();
q.pop();
for (int dest : rev_links[node])
{
out_deg[dest] --;
if (out_deg[dest] == 0)
q.push(dest);
}
}
}
return start;
}
int Clear() {
queue < int > q;
for (int i = 0; i < n; i ++)
{
if (!out_deg[i])
q.push(i);
}
int start = 0;
while (!q.empty() || out_deg[start] <= 1)
{
if (!q.empty())
{
int node = q.front();
q.pop();
seen[node] = -1;
for (int dest : rev_links[node])
if (!(-- out_deg[dest]))
q.push(dest);
}
while(out_deg[start] <= 1)
{
seen[start] = -1;
out_deg[start] = 0;
for (int dest : rev_links[start])
{
out_deg[dest] --;
if (out_deg[dest] == 0)
q.push(dest);
}
pair < int, int > nxt = make_pair(-1, 0);
for (pair < int, int > cur : graph[start])
{
if (seen[cur.first] != -1)
{
nxt = cur;
break;
}
}
if (nxt.first == -1)
return -1;
start = nxt.first;
path[2].push_back(nxt.second);
}
}
return start;
}
std::variant<bool, std::vector<int>> find_journey(int N, int M, std::vector<int> U, std::vector<int> V)
{
n = N;
m = M;
for (int i = 0; i < M; i ++)
{
links[U[i]].push_back(V[i]);
rev_links[V[i]].push_back(U[i]);
in_deg[V[i]] ++;
out_deg[U[i]] ++;
graph[U[i]].push_back({V[i], i});
}
int root = Clear();
if (root < 0)
return false;
///cout << " root " << root << endl;
///dfs(start, 0, start);
return true;
}
