Submission #1060227

#	Time	Username	Problem	Language	Result	Execution time	Memory
1060227		gyg	Cat in a tree (BOI17_catinatree)	C++17	100 / 100	832 ms	255712 KiB

catinatree

#include <bits/stdc++.h>
using namespace std;
using pii = pair<int, int>;
const int MAX_N = 2e5 + 5;

int n, k;
array<vector<int>, MAX_N> adj;

array<int, MAX_N> depth;
void dfs(int u = 1, int par = -1) {
    for (int v : adj[u])
        if (v != par) depth[v] = depth[u] + 1, dfs(v, u);
}

array<int, MAX_N> siz;
array<bool, MAX_N> del;
void update_siz(int u, int par = -1) {
    siz[u] = 1;
    for (int v : adj[u]) {
        if (v == par) continue;
        if (del[v]) continue;
        update_siz(v, u), siz[u] += siz[v];
    }
}
int find_cent(int u, int tot_siz, int par = -1) {
    for (int v : adj[u]) {
        if (v == par) continue;
        if (del[v]) continue;
        if (siz[v] * 2 > tot_siz) return find_cent(v, tot_siz, u);
    }
    return u;
}

array<vector<int>, MAX_N> ancs; // Reversed
array<unordered_map<int, int>, MAX_N> dist;
array<vector<pii>, MAX_N> dists; // Smallest last for popping purposes
void find_dists(int u, int d, int par, int cent) {
    dist[cent][u] = d, dists[cent].push_back({d, u});
    ancs[u].push_back(cent);
    for (int v : adj[u]) {
        if (v == par) continue;
        if (del[v]) continue;
        find_dists(v, d + 1, u, cent);
    }
}

void build(int u = 1) {
    update_siz(u);
    int cent = find_cent(u, siz[u]);
    
    find_dists(cent, 0, -1, cent);
    sort(dists[cent].rbegin(), dists[cent].rend());

    del[cent] = true;
    for (int v : adj[cent])
        if (!del[v]) build(v);
}

array<bool, MAX_N> used;
void update_used(int u) {
    for (int v : ancs[u]) {
        int d = k - dist[v][u];
        while (dists[v].size() && (dists[v].back().first < d || used[dists[v].back().second])) { // sus
            used[dists[v].back().second] = true;
            dists[v].pop_back();
        } 
    }
}

vector<pii> order;
int comp() {
    for (int u = 1; u <= n; u++) order.push_back({depth[u], u}); 
    sort(order.rbegin(), order.rend());   
    
    int ans = 0;
    for (pii x : order) {
        int u = x.second;
        if (used[u]) continue;
        ans++;
        update_used(u);
        // cout << "DELETED: " << u << endl;
        // cout << "LEFT: ";
        // for (int v = 1; v <= n; v++)
        //     if (!used[v]) cout << v << " ";
        // cout << endl;
    }
    return ans;
}

int main() {
    // freopen("cat.in", "r", stdin);
    cin >> n >> k; 
    for (int u = 2; u <= n; u++) {
        int v; cin >> v; v++;
        adj[u].push_back(v), adj[v].push_back(u);
    }

    dfs();
    build();

    // for (int u = 1; u <= n; u++) {
    //     cout << u << ": ";
    //     for (pii x : dists[u][u]) cout << x.second << "," << x.first << " ";
    //     cout << endl; 
    // }
 
    int ans = comp();
    cout << ans << '\n';
}

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