Submission #931766

#	Time	Username	Problem	Language	Result	Execution time	Memory
931766		vjudge1	Network (BOI15_net)	C++17	0 / 100	1 ms	428 KiB

net

#include <bits/stdc++.h>
#define F first
#define S second
#define pb push_back
using namespace std;

const int N = 2000 + 10, LG = 20;

int n, h[N], par[N][LG];
bool mark[N];
vector<int> G[N], vec;
vector<pair<int, pair<int, int>>> vec1;

void dfs(int v = 1, int p = 0) {
    par[v][0] = p;
    if (G[v].size() == 1) vec.pb(v);
    for (auto e : G[v])
        if (e ^ p) {
            h[e] = h[v] + 1;
            dfs(e, v);
        }
}

int lca(int u, int v) {
    if (u == v) return u;
    if (h[u] < h[v]) swap(u, v);
    int k = h[u] - h[v];
    for (int lg = 0; lg < LG; lg++)
        if (k & (1 << lg)) u = par[u][lg];
    if (u == v) return v;
    for (int lg = LG - 1; lg >= 0; lg--)
        if (par[u][lg] != par[v][lg])  u = par[u][lg], v = par[v][lg];
    return par[u][0];
}

int main() {
    ios:: sync_with_stdio(0), cin.tie(0);
    cin >> n;
    for (int i = 0; i < n - 1; i++) {
        int u, v;
        cin >> u >> v;
        G[u].pb(v);
        G[v].pb(u);
    }
    dfs();
    for (int lg = 1; lg < LG; lg++)
        for (int i = 1; i <= n; i++)
            par[i][lg] = par[par[i][lg - 1]][lg - 1];
    for (int i = 0; i < (int)vec.size(); i++) 
        for (int j = i + 1; j < (int)vec.size(); j++) {
            int l = lca(vec[i], vec[j]);
            vec1.pb({h[vec[i]] + h[vec[j]] - 2 * h[l], {vec[i], vec[j]}});
        }
    sort(vec1.begin(), vec1.end());
    reverse(vec1.begin(), vec1.end());
    cout << ((int)vec.size() + 1) / 2 << endl;
    int cnt = 0;
    for (auto e : vec1) {
        if (cnt == (int) vec.size()) return 0;
        if (!mark[e.S.F] && !mark[e.S.S]) {
            cout << e.S.F << ' ' << e.S.S << endl;
            mark[e.S.F] = mark[e.S.S] = true, cnt += 2;
        }
        else if ((int)vec.size() & 1)
            if (cnt + 1 == (int)vec.size()) {
                if (!mark[e.S.S]) return cout << vec1[0].S.F << ' ' << e.S.S << endl, 0;
                else if (!mark[e.S.F]) return cout << vec1[0].S.F << ' ' << e.S.F << endl, 0;
            }
    }
}

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