Submission #931699

#	Time	Username	Problem	Language	Result	Execution time	Memory
931699		vjudge1	Network (BOI15_net)	C++17	0 / 100	1 ms	500 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) {
    int res = h[u] + h[v];
    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];
    for (int lg = LG - 1; lg >= 0; lg--)
        if (par[u][lg] ^ par[v][lg])  u = par[u][lg], v = par[v][lg];
    return res - 2 * h[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 i = 0; i < (int)vec.size(); i++) 
        for (int j = i + 1; j < (int)vec.size(); j++)
            vec1.pb({lca(vec[i], vec[j]), {vec[i], vec[j]}});
    sort(vec1.rbegin(), vec1.rend());
    cout << (vec.size() + 1) / 2 << endl;
    for (auto e : vec1)
        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;
        }
        else if (vec.size() & 1) {
            int cnt = 0;
            for (int i = 1; i <= n; i++)
                if (!mark[i]) cnt++;
            if (cnt + 1 == vec.size()) {
                if (!mark[e.S.S]) {
                    cout << vec1[0].S.F << ' ' << e.S.S;
                    break;
                }
                if (!mark[e.S.F]) {
                    cout << vec1[0].S.F << ' ' << e.S.F;
                    break;
                }
            }
        }
}

Compilation message (stderr)

net.cpp: In function 'int main()':
net.cpp:59:25: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   59 |             if (cnt + 1 == vec.size()) {
      |                 ~~~~~~~~^~~~~~~~~~~~~

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