Submission #583481

#	Time	Username	Problem	Language	Result	Execution time	Memory
583481		600Mihnea	Mousetrap (CEOI17_mousetrap)	C++17	45 / 100	1116 ms	64080 KiB

mousetrap

#include <bits/stdc++.h>

using namespace std;

const int N = (int) 1e6 + 7;
int n, tr, mo;
vector<int> g[N];
int par[N], cost[N];
bool vis[N];

void build_tree(int a, int p = -1) {
  {
    vector<int> Kids;
    for (auto &b : g[a]) {
      if (b != p) {
        Kids.push_back(b);
      }
    }
    g[a] = Kids;
  }
  for (auto &b : g[a]) {
    build_tree(b, a);
  }
  par[a] = p;
}

void build_cost(int a) {
  if (!vis[a] && a != mo) {
    cost[a] += (int) g[a].size();
  }
  for (auto &b : g[a]) {
    cost[b] += cost[a];
    build_cost(b);
  }
}

void dfs(int a) {
  for (auto &b : g[a]) {
    dfs(b);
  }
  if ((int) g[a].size() == 0) {
    return;
  }
  if ((int) g[a].size() == 1) {
    return;
  }
  assert((int) g[a].size() >= 2);
  vector<int> Values;
  for (auto &b : g[a]) {
    Values.push_back(cost[b]);
  }
  sort(Values.rbegin(), Values.rend());
  assert((int) Values.size() >= 2);
  cost[a] = Values[1];
}

bool ok(int limit) {
  vector<pair<int, int>> guys;
  int dist = 1;
  guys.push_back({0, cost[mo]});
  int v = mo;
  int sum = 0;
  {
    for (int i = 1; i <= n; i++) {
      if ((vis[i] && i != tr) || i == mo) {
        sum += (int) g[i].size() - 1;
      }
    }
    sum++;

  }
  if (sum > limit) {
    return 0;
  }
  int cnt = 0;
  while (v != tr) {
    int skip = 0;
    for (auto &oth : g[v]) {
      if (!vis[oth]) {
        skip += (sum + cost[oth] <= limit);
        cnt += (sum + cost[oth] > limit);
      }
    }
    sum -= skip;
    if (cnt > dist) return 0;
    v = par[v];
    dist++;
  }
  return 1;
}

signed main() {
  ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);

///  freopen ("input.txt", "r", stdin);

  cin >> n >> tr >> mo;

  for (int i = 1; i < n; i++) {
    int a, b;
    cin >> a >> b;
    g[a].push_back(b);
    g[b].push_back(a);
  }
  build_tree(tr);
  {
    int current = par[mo];
    while (current != tr) {
      vis[current] = 1;
      current = par[current];
    }
    vis[current] = 1;
  }
  build_cost(tr);
  dfs(tr);


  int low = 0, high = (int) 1e9 + 7, sol = -1;

  while (low <= high) {
    int mid = (low + high) / 2;
    if (ok(mid)) {
      sol = mid;
      high = mid - 1;
    } else {
      low = mid + 1;
    }
  }

  assert(sol != -1);

  cout << sol << "\n";

}

• Subtask 1: 20 / 20
• Subtask 2: 25 / 25
• Subtask 3: 0 / 20
• Subtask 4: 0 / 35