Submission #632577

#	Time	Username	Problem	Language	Result	Execution time	Memory
632577		stevancv	Network (BOI15_net)	C++14	100 / 100	579 ms	77144 KiB

net

#include <bits/stdc++.h>
#define ll long long
#define ld long double
#define sp ' '
#define en '\n'
#define smin(a, b) a = min(a, b)
#define smax(a, b) a = max(a, b)
using namespace std;
const int N = 5e5 + 2;
const int mod = 1e9 + 7;
vector<int> g[N];
int szl[N];
vector<vector<int>> all;
void Dfs(int s, int e) {
    for (int u : g[s]) {
        if (u != e) {
            Dfs(u, s);
            szl[s] += szl[u];
        }
    }
    smax(szl[s], 1);
}
int Dfs1(int s, int e, int t) {
    for (int u : g[s]) {
        if (u == e) continue;
        if (2 * szl[u] > t) return Dfs1(u, s, t);
    }
    return s;
}
void Dfs2(int s, int e, int r) {
    if (g[s].size() == 1) all.back().push_back(s);
    for (int u : g[s]) {
        if (u == e) continue;
        if (s == r) all.push_back({});
        Dfs2(u, s, r);
    }
}
int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int n;
    cin >> n;
    for (int i = 0; i < n - 1; i++) {
        int u, v; cin >> u >> v;
        g[u].push_back(v);
        g[v].push_back(u);
    }
    int root = 1;
    for (int i = 1; i <= n; i++) if (g[i].size() > g[root].size()) root = i;
    Dfs(root, 0);
    root = Dfs1(root, 0, szl[root]);
    Dfs2(root, 0, root);
    int sz = all.size();
    priority_queue<pair<int, int>> pq;
    vector<array<int, 2>> ans;
    for (int i = 0; i < sz; i++) pq.push({all[i].size(), i});
    int who = -1;
    while (pq.size() > 1) {
        int x = pq.top().second; pq.pop();
        int y = pq.top().second; pq.pop();
        who = all[x].back();
        ans.push_back({all[x].back(), all[y].back()});
        all[x].pop_back(); all[y].pop_back();
        if (all[x].size() > 0) pq.push({all[x].size(), x});
        if (all[y].size() > 0) pq.push({all[y].size(), y});
    }
    if (pq.size() == 1) ans.push_back({who, all[pq.top().second].back()});
    cout << ans.size() << en;
    for (int i = 0; i < ans.size(); i++) {
        cout << ans[i][0] << sp << ans[i][1] << en;
    }
    return 0;
}

Compilation message (stderr)

net.cpp: In function 'int main()':
net.cpp:70:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::array<int, 2> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   70 |     for (int i = 0; i < ans.size(); i++) {
      |                     ~~^~~~~~~~~~~~

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