Submission #1215222

#	Time	Username	Problem	Language	Result	Execution time	Memory
1215222		madamadam3	Network (BOI15_net)	C++20	18 / 100	20 ms	3140 KiB

net

#include <bits/stdc++.h>
using namespace std;

void tarjan(int u, int p, vector<vector<int>> &adj, vector<int> &tin, vector<int> &low, vector<bool> &vis, int &timer, bool &is_bridge) {
    vis[u] = true;
    tin[u] = low[u] = timer++;

    for (auto &v : adj[u]) {
        if (v == p) continue;
        if (vis[v]) {
            low[u] = min(low[u], tin[v]);
            continue;
        }
        tarjan(v, u, adj, tin, low, vis, timer, is_bridge);
        low[u] = min(low[u], low[v]);
        is_bridge = is_bridge || (low[v] > tin[u]);
    }
}

bool has_bridge(vector<vector<bool>> &adjmat) {
    int n = adjmat.size();
    vector<vector<int>> adj(n, vector<int>());
    for (int i = 0; i < n; i++) {
        for (int j = i+1; j < n; j++) {
            if (adjmat[i][j] != 1) continue;
            adj[i].push_back(j);
            adj[j].push_back(i);
        }
    }

    vector<int> tin(n, -1); vector<int> low(n, -1); vector<bool> vis(n, false); int timer = 0; bool is_bridge = false;
    tarjan(0, 0, adj, tin, low, vis, timer, is_bridge);
    return is_bridge;
}

int n;
vector<vector<int>> adj; vector<pair<int, int>> edges;
vector<int> sz, leaves;

void get_subtree_size(int u, int p) {
    sz[u] = 1;
    for (auto &v : adj[u]) {
        if (v == p) continue;
        get_subtree_size(v, u);
        sz[u] += sz[v];
    }

    if (sz[u] == 1 && p != -1) leaves.push_back(u);
}

int find_centroid(int u, int p) {
    for (auto &v : adj[u]) {
        if (v == p) continue;
        if (sz[v] * 2 > n) return find_centroid(v, u);
    }

    return u;
}

uint64_t next_subset(uint64_t x) {
    uint64_t c = x & -x;     
    uint64_t r = x + c;        
    return (((r ^ x) >> 2) / c) | r;
}

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

    cin >> n;
    adj.assign(n, vector<int>()); sz.assign(n, -1);
    for (int i = 0; i < n - 1; i++) {
        int u, v; cin >> u >> v;
        u--; v--;
        adj[u].push_back(v);
        adj[v].push_back(u);
        edges.push_back({u, v});
    }
    
    get_subtree_size(0, -1);
    int centre = find_centroid(0, -1);
    leaves.clear(); get_subtree_size(centre, -1);

    int n_edges = int(leaves.size() - 1) * int(leaves.size()) / 2;
    int best_mask = 0;

    for (int k = 1; k <= n_edges; k++) {
        int mask = (1 << k) - 1;
        while (mask < (1 << n_edges)) {
            vector<vector<bool>> adjmat(n, vector<bool>(n, false));
            for (auto &el : edges) adjmat[el.first][el.second] = adjmat[el.second][el.first] = true;
            int cur_bit = 0;
            for (int i = 0; i < leaves.size(); i++) {
                for (int j = i + 1; j < leaves.size(); j++) {
                    adjmat[leaves[i]][leaves[j]] = adjmat[leaves[j]][leaves[i]] = mask & (1 << cur_bit);
                    cur_bit++;
                }
            }

            if (!has_bridge(adjmat)) {
                best_mask = mask;
                break;
            }

            mask = next_subset(mask);
        }

        if (best_mask != 0) break;
    }

    vector<pair<int, int>> new_edges;
    int cur_bit = 0;
    for (int i = 0; i < leaves.size(); i++) {
        for (int j = i + 1; j < leaves.size(); j++) {
            if (best_mask & (1 << cur_bit)) {
                new_edges.push_back({leaves[i]+1, leaves[j]+1});
            }
            cur_bit++;
        }
    }

    cout << new_edges.size() << "\n";
    for (auto &el : new_edges) {
        cout << el.first << " " << el.second << "\n";
    }
    return 0;
}

• Subtask 1: 18 / 18
• Subtask 2: 0 / 45
• Subtask 3: 0 / 37