Submission #583413

#	Time	Username	Problem	Language	Result	Execution time	Memory
583413		TheLimePixel	Network (BOI15_net)	C++17	100 / 100	613 ms	53980 KiB

net

#include "bits/stdc++.h"

using namespace std;

vector<vector<int>> graph;
vector<int> leaves_from, leaves, res;
int leaf_count, res_count;

void count_leaves(int node, int parent) {
    if (graph[node].size() == 1)
       leaves_from[node] = 1;

    for (auto adj : graph[node]) {
        if (adj == parent) 
            continue; 

        count_leaves(adj, node);
        leaves_from[node] += leaves_from[adj];
    }
} 

int find_mid(int node) {
    int parent = 0;

    for (auto adj : graph[node])
        if (adj != parent && leaves_from[adj] > leaf_count / 2) {
            parent = node;
            node = adj;
        }

  return node;
}

void add_leaves(int node, int parent) {
    if (graph[node].size() == 1)
        leaves.push_back(node);

    for (auto adj : graph[node])
        if (adj != parent)
            add_leaves(adj, node);
}

int split(int n) {
    count_leaves(1, 0);

    int mid = find_mid(1);

    for (auto adj : graph[mid])
        add_leaves(adj, mid);

    return mid;
}

void run() {
    int n;
    cin >> n;

    if (n == 1) {
        cout << 0;
        return;
    }

    if (n == 2) {
        cout << "1\n2";
        return;
    }

    graph = vector<vector<int>>(n + 1);
    leaves_from = vector<int>(n + 1);

    for (int i = 0; i < n - 1; i++) {
        int a, b;
        cin >> a >> b;
        graph[a].push_back(b);
        graph[b].push_back(a);
    }

    for (int i = 1; i <= n; i++)
        if (graph[i].size() == 1)
            leaf_count++;

    res_count = (leaf_count + 1) / 2;
    cout << res_count;
    leaves.reserve(2 * res_count);
    res.resize(2 * res_count);

    split(n);

    int i = 0;

    for (auto leaf : leaves) {
        res[i] = leaf;
        i += 2;
        if (i == res_count * 2)
            i = 1;
    }

    if (!res.empty() && res.back() == 0)
        res.back() = leaves[0];

    for (int i = 0; i < res_count; i++)
        cout << '\n' << res[2 * i] << ' ' << res[2 * i + 1];
}

auto main() -> int {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    run();
}

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