Submission #656126

#	Time	Username	Problem	Language	Result	Execution time	Memory
656126		Kahou	Network (BOI15_net)	C++14	100 / 100	557 ms	90312 KiB

net

#include<bits/stdc++.h>
using namespace std;
#define F first
#define S second
#define endl '\n'
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const int N = 5e5 + 50;
int n, sub[N], k, par[N], t;
vector<int> adj[N];
vector<int> vc[N];
set<pii> st;
vector<pii> out;

void dfs(int u, int p) {
        par[u] = p;
        for (int v:adj[u]) {
                if (v == p) continue;
                dfs(v, u);
                sub[u] += sub[v];
        }
}

void dfs2(int u, int p) {
        if (adj[u].size() == 1) {
                vc[t].push_back(u);
        }
        for (int v:adj[u]) {
                if (v == p) continue;
                dfs2(v, u);
        }
}

void solve() {
        cin >> n;
        for (int i = 1; i <= n-1; i++) {
                int u, v;
                cin >> u >> v;
                adj[u].push_back(v);
                adj[v].push_back(u);
        }
        for (int u = 1; u <= n; u++) {
                if (adj[u].size() == 1) {
                        k++;
                        sub[u] = 1;
                }
        }
        dfs(1, 0);
        int r = 0;
        for (int u = 1; u <= n; u++) {
                if (adj[u].size() == 1) continue;
                bool flg = 1;
                for (int v:adj[u]) {
                        if (v == par[u]) flg = flg&&(k-sub[u] <= k/2);
                        else flg = flg&&(sub[v] <= k/2);
                }
                if (flg) {
                        r = u;
                        break;
                }
        }
        for (int u:adj[r]) {
                t++;
                dfs2(u, r);
        }
        for (int i = 1; i <= t; i++) {
                st.insert({-vc[i].size(), i});
        }
        while (st.size() >= 2) {
                auto it = st.begin();
                int A = it->S; it++;
                int B = it->S;

                out.push_back({vc[A].back(), vc[B].back()});

                st.erase({-vc[A].size(), A});
                st.erase({-vc[B].size(), B});

                vc[A].pop_back();
                vc[B].pop_back();
                if (!vc[A].empty()) st.insert({-vc[A].size(), A});
                if (!vc[B].empty()) st.insert({-vc[B].size(), B});
        }
        if (st.size()) {
                int A = st.begin()->S;
                int u = vc[A].back();
                out.push_back({u, r});
        }

        cout << out.size() << endl;
        for (pii x:out) {
                cout << x.F << ' ' << x.S << endl;
        }
}

int main() {
        ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
        solve();
        return 0;
}

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