Submission #1074500

#	Time	Username	Problem	Language	Result	Execution time	Memory
1074500		fv3	Thousands Islands (IOI22_islands)	C++17	24 / 100	90 ms	8732 KiB

islands

#include "islands.h"
#include <bits/stdc++.h>

using namespace std;
vector<vector<pair<int, int>>> adj;
vector<int> visited;
bool ok;

vector<int> find_cycle(int s, set<int>&path)
{
  bool skip = false;
  vector<int> cycle;
  int i = s;
  do
  {
    for (auto node : adj[i])
    {
      if (!path.count(node.first)) continue;
      cycle.push_back(node.second);
      i = node.first;
      break;
    }
  }
  while (i != s);

  const int sz = cycle.size();
  for (int i = 0; i < sz; i++)
    cycle.push_back(cycle[i] ^ 1);
  for (int i = 2 * sz - 1 ; i >= 0; i--)
    cycle.push_back(cycle[i] ^ 1);

  return cycle;
}

void DFS(int index, set<int>&path, vector<int>&v)
{
  visited[index] = 1;
  path.insert(index);

  for (auto node : adj[index])
  {
    if (path.count(node.first))
    {
      ok = true;
      vector<int> cycle = find_cycle(node.first, path);
      const int sz = cycle.size();
      for (int i = 0; i < sz / 4 - 1; i++)
        v.pop_back();

      v.insert(v.end(), cycle.begin(), cycle.end());
      for (int i = v.size() - sz - 1; i >= 0; i--)
        v.push_back(v[i]);
      return;
    }
    if (visited[node.first]) continue; 
    v.push_back(node.second);
    DFS(node.first, path, v);
    if (ok) return;
    v.pop_back();
  }
  if (ok) return;

  path.erase(index);
}

variant<bool, vector<int>> find_journey(int N, int M, vector<int> U, vector<int> V) 
{
  adj = vector<vector<pair<int, int>>>(N);

  set<pair<int, int>> s;
  for (int i = 0; i < M; i += 2)
  {
    if (s.count({U[i], V[i]})) continue;
    adj[U[i]].push_back({V[i], i});
    s.insert({U[i], V[i]});
  }

  ok = false;
  visited = vector<int>(N);
  set<int> path;
  vector<int> res;
  DFS(0, path, res);

  if (ok)
    return res;
  return false;
}

Compilation message (stderr)

islands.cpp: In function 'std::vector<int> find_cycle(int, std::set<int>&)':
islands.cpp:11:8: warning: unused variable 'skip' [-Wunused-variable]
   11 |   bool skip = false;
      |        ^~~~

• Subtask 1: 0 / 5
• Subtask 2: 0 / 5
• Subtask 3: 0 / 21
• Subtask 4: 24 / 24
• Subtask 5: 0 / 45