Submission #1174475

#	Time	Username	Problem	Language	Result	Execution time	Memory
1174475		_ncng.nyr	Cat in a tree (BOI17_catinatree)	C++20	100 / 100	598 ms	50504 KiB

catinatree

#include<bits/stdc++.h>

using namespace std;

const int N = 2e5 + 5;

int n, D, it;
int in[N], out[N], h[N];
int up[20][N];
vector<int> ad[N];

void init(int u, int p) {
  in[u] = ++it;
  if(u == 1) for(int i = 0; i <= 18; ++i) up[i][u] = u;
  else for(int i = 1; i <= 18; ++i) up[i][u] = up[i - 1][up[i - 1][u]];

  for(auto v : ad[u]) {
    if(v == p) continue;
    up[0][v] = u;
    h[v] = h[u] + 1;
    init(v, u);
  }
  out[u] = it;
}

int anc(int u, int v) {
  return (in[u] <= in[v] && out[v] <= out[u]);
}

int lca(int u, int v) {
  if(anc(u, v)) return u;
  int ret = u;
  for(int i = 18; i >= 0; --i)
    if(anc(up[i][u], v)) ret = up[i][u];
    else u = up[i][u];
  return ret;
}

int dist(int u, int v) {
  return h[u] + h[v] - 2 * h[lca(u, v)];
}

namespace sub1 {
  int ans = 0;
  void solve() {
    for(int mask = 0; mask < (1 << n); ++mask) {
      bool ok = 1;
      for(int i = 1; i <= n; ++i)
        for(int j = 1; j < i; ++j)
        if(((mask >> i - 1) & 1) && ((mask >> j - 1) & 1) && dist(i, j) < D) { ok = 0; break; }

      if(ok) ans = max(ans, __builtin_popcount(mask));
    }

    cout << ans;
  }
}

namespace sub2 {
  int ans = 0;
  void solve() {
    vector<int> v;
    for(int i = 1; i <= n; ++i) v.push_back(i);

    while(v.size()) {
      ++ans;
      int mx = 0;
      for(auto u : v)
        if(h[u] > h[mx]) mx = u;

      vector<int> nxt;
      for(auto u : v)
        if(dist(u, mx) >= D) nxt.push_back(u);

      v = nxt;
    }

    cout << ans;
  }
}

// lấy dần các con ở độ sâu thấp nhất
namespace sub3 {
  #define ii pair<int, int>
  priority_queue<ii, vector<ii>, greater<ii>> q[N];

  void dfs(int u, int p) {
    for(auto v : ad[u]) {
      if(v == p) continue;
      dfs(v, u);

      while(q[u].size() && q[v].size() &&
          q[u].top().first + q[v].top().first - 2 * h[lca(q[u].top().second, q[v].top().second)] < D) {

          if(q[u].top().first < q[v].top().first) q[u].pop();
          else q[v].pop();
      }

      if(q[u].size() < q[v].size()) swap(q[u], q[v]);
      while(q[v].size()) q[u].push(q[v].top()), q[v].pop();
    }

    if(q[u].empty() || q[u].top().first + h[u] - 2 * h[lca(q[u].top().second, u)] >= D) q[u].emplace(h[u], u);
  }

  void solve() {
    dfs(1, 0);
    cout << q[1].size() << '\n';
  }
}

int32_t main() {
  cin.tie(0)->sync_with_stdio(0);

  cin >> n >> D;

  for(int i = 2; i <= n; ++i) {
    int u; cin >> u; ++u;
    ad[u].push_back(i);
    ad[i].push_back(u);
  }

  init(1, 0);

  if(n <= 18) sub1 :: solve();
  else if(n <= 1'500) sub2 :: solve();
  else sub3 :: solve();
}

• Subtask 1: 11 / 11
• Subtask 2: 40 / 40
• Subtask 3: 49 / 49