Submission #655948

#	Time	Username	Problem	Language	Result	Execution time	Memory
655948		ParsaS	Network (BOI15_net)	C++17	0 / 100	4 ms	5028 KiB

net

// In the name of God
#include<bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define pb push_back
#define mp make_pair
typedef long long ll;

const int N = 2e5 + 5, MOD = 1e9 + 7;
int n, tin[N], T;
vector<int> adj[N];

void dfs(int v, int p) {
    tin[v] = ++T;
    for (auto u : adj[v])
        if (u != p)
            dfs(u, v);
}

void solve() {
    cin >> n;
    for (int i = 0; i < n - 1; i++) {
        int v, u; cin >> v >> u;
        adj[v].pb(u), adj[u].pb(v);
    }
    int root = 1;
    for (int i = 1; i <= n; i++) {
        if (adj[i].size() >= 2)
            root = i;
    }

    dfs(root, 0);
    vector<pair<int, int> > vec, ans;
    for (int i = 1; i <= n; i++) {
        if (adj[i].size() == 1)
            vec.pb(mp(tin[i], i));
    }
    sort(vec.begin(), vec.end());
    if (vec.size() % 2) {
        ans.pb(mp(vec.back().se, root));
        vec.pop_back();
    }
    for (int i = 0; i < vec.size() / 2; i++)
        ans.pb(mp(vec[i].se, vec[i + vec.size() / 2].se));
    cout << ans.size() << '\n';
    for (auto [v, u] : ans)
        cout << v << ' ' << u << '\n';
}

int32_t main() {
    ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
    solve();
    return 0;
}

Compilation message (stderr)

net.cpp: In function 'void solve()':
net.cpp:44:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::pair<int, int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   44 |     for (int i = 0; i < vec.size() / 2; i++)
      |                     ~~^~~~~~~~~~~~~~~~

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