Submission #548272

#	Time	Username	Problem	Language	Result	Execution time	Memory
548272		lorenzoferrari	Islands (IOI08_islands)	C++17	54 / 100	931 ms	131072 KiB

islands

#include <bits/stdc++.h>
#pragma GCC optimize ("O3")
using namespace std;
using LL = long long;

#define watch(x) cerr << (#x) << " is " << x << endl

vector<bool> vis, cycle;
vector<array<int, 2>> prv, nxt;
vector<LL> dl, dr;
vector<LL> f;

LL max_path(const vector<int>& c, const vector<vector<array<LL, 3>>>& adj) {
  auto find_cycle = [&](auto&& self, int v, int pid = -1) -> bool {
    if (vis[v]) return cycle[v] = true;
    vis[v] = true;
    for (auto [u, w, eid] : adj[v]) {
      if (eid == pid) continue;
      bool works = self(self, u, eid);
      if (works) {
        nxt[v] = {u, w};
        prv[u] = {v, w};
        return cycle[v] ? false : cycle[v] = true;
      }
    }
    return false;
  };
  find_cycle(find_cycle, c[0]);
  auto path_in_tree = [&](auto&& self, int v, int p = -1) -> LL {
    LL ans = 0;
    for (auto [u, w, eid] : adj[v]) {
      if (u == p || cycle[u]) continue;
      ans = max(ans, w + self(self, u, v));
    }
    return ans;
  };
  int start = -1;
  for (int i : c) {
    if (cycle[i]) {
      start = i;
      f[i] = path_in_tree(path_in_tree, i);
    }
  }
  multiset<LL> L, R;
  {
    int v = start;
    int u = start;
    dl[v] = dr[u] = 0;
    do {
      auto [nv, lw] = nxt[v];
      auto [nu, rw] = prv[u];
      if (v != start) L.insert(f[v] + dl[v]);
      if (u != start) R.insert(f[u] + dr[u]);
      if (nv != start) dl[nv] = dl[v] + lw;
      dr[nu] = dr[u] + rw;
      v = nv;
      u = nu;
    } while (v != start);
  }
  assert(dl[start] == 0);
  LL len = dr[start];
  LL bias = 0;
  LL ans = 0;
  {
    int v = start;
    do {
      // watch(v);
      // watch(bias);
      // cerr << "L: "; for (LL i : L) cerr << i << " "; cerr << endl;
      // cerr << "R: "; for (LL i : R) cerr << i << " "; cerr << endl;
      // cerr << endl;
      ans = max(ans, *prev(L.end()) - bias + f[v]);
      ans = max(ans, *prev(R.end()) + bias + f[v]);
      int nv = nxt[v][0];
      if (nv == start) break;
      bias += nxt[v][1];
      L.insert(f[v] + dl[v] + len);
      R.insert(f[v] + dr[v] - len);
      if (L.count(f[nv] + dl[nv])) L.erase(L.find(f[nv] + dl[nv]));
      if (R.count(f[nv] + dr[nv])) R.erase(R.find(f[nv] + dr[nv]));
      v = nv;
    } while (v != start);
  }
  return ans;
}

int main() {
#ifdef LORENZO
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n; cin >> n;
  vector<vector<array<LL, 3>>> adj(n);
  for (int i = 0; i < n; ++i) {
    int a; cin >> a; --a;
    int w; cin >> w;
    adj[i].push_back({a, w, i});
    adj[a].push_back({i, w, i});
  }
  vis.assign(n, false);
  vector<vector<int>> cc;
  auto dfs = [&](auto&& self, int v) -> void {
    vis[v] = true;
    cc.back().push_back(v);
    for (auto [u, _, __] : adj[v]) {
      if (!vis[u]) {
        self(self, u);
      }
    }
  };
  for (int i = 0; i < n; ++i) {
    if (!vis[i]) {
      cc.push_back(vector<int>(0));
      dfs(dfs, i);
    }
  }
  f.assign(n, 0);
  dl.assign(n, 1e18);
  dr.assign(n, 1e18);
  prv.assign(n, {-1,-1});
  nxt.assign(n, {-1,-1});
  vis.assign(n, false);
  cycle.assign(n, false);
  LL ans = 0;
  for (const auto& c : cc) {
    ans += max_path(c, adj);
  }
  cout << ans << "\n";
}

Compilation message (stderr)

islands.cpp: In instantiation of 'max_path(const std::vector<int>&, const std::vector<std::vector<std::array<long long int, 3> > >&)::<lambda(auto:23&&, int, int)> [with auto:23 = max_path(const std::vector<int>&, const std::vector<std::vector<std::array<long long int, 3> > >&)::<lambda(auto:23&&, int, int)>&]':
islands.cpp:28:31:   required from here
islands.cpp:21:16: warning: narrowing conversion of 'u' from 'std::tuple_element<0, std::array<long long int, 3> >::type' {aka 'long long int'} to 'int' [-Wnarrowing]
   21 |         nxt[v] = {u, w};
islands.cpp:21:16: warning: narrowing conversion of 'w' from 'std::tuple_element<1, std::array<long long int, 3> >::type' {aka 'long long int'} to 'int' [-Wnarrowing]
islands.cpp:22:16: warning: narrowing conversion of 'w' from 'std::tuple_element<1, std::array<long long int, 3> >::type' {aka 'long long int'} to 'int' [-Wnarrowing]
   22 |         prv[u] = {v, w};
      |         ~~~~~~~^~~~~~~~

• Subtask 1: 27 / 33
• Subtask 2: 3 / 3
• Subtask 3: 4 / 4
• Subtask 4: 0 / 10
• Subtask 5: 10 / 10
• Subtask 6: 10 / 10
• Subtask 7: 0 / 10
• Subtask 8: 0 / 10
• Subtask 9: 0 / 10