Submission #390058

#	Time	Username	Problem	Language	Result	Execution time	Memory
390058		BThero	Network (BOI15_net)	C++17	100 / 100	688 ms	58172 KiB

net

// chrono::system_clock::now().time_since_epoch().count()
#include <bits/stdc++.h>

#define pb push_back
#define eb emplace_back
#define mp make_pair
#define fi first
#define se second
#define all(x) (x).begin(), (x).end()
#define sz(x) (int)(x).size()
#define rep(i, a, b) for (int i = (a); i < (b); ++i)
#define debug(x) cerr << #x << " = " << x << endl

using namespace std;

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

const int MAXN = (int)5e5 + 5;

vi adj[MAXN], leaf;
int sub[MAXN];
int n, m;

void dfs(int v, int pr = -1) {
    sub[v] = (sz(adj[v]) == 1);

    for (int to : adj[v]) {
        if (pr != to) {
            dfs(to, v);
            sub[v] += sub[to];
        }
    }
}

int cen(int v, int lim, int pr = -1) {
    for (int to : adj[v]) {
        if (to != pr && sub[to] > lim / 2) {
            return cen(to, lim, v);
        }
    }

    return v;
}

void dfsCol(int v, int pr, int c) {
    if (sz(adj[v]) == 1) {
        leaf.pb(v);
    }

    for (int to : adj[v]) {
        if (to != pr) {
            dfsCol(to, v, c);
        }
    }
}

void solve() {
    cin >> n;

    rep (i, 1, n) {
        int u, v;
        cin >> u >> v;
        adj[u].pb(v);
        adj[v].pb(u);
    }

    dfs(1);
    int tot = sub[1];
    int root = cen(1, tot);

    for (int to : adj[root]) {
        dfsCol(to, root, ++m);
    }
    
    cout << (tot + 1) / 2 << '\n';

    if (tot % 2 == 0) {
        for (int i = 0; i < tot / 2; i++) {
            cout << leaf[i] << ' ' << leaf[i + tot / 2] << '\n';
        }
    }
    else {
        for (int i = 0; i <= tot / 2; i++) {
            cout << leaf[i] << ' ' << leaf[i + tot / 2] << '\n';
        }
    }
}

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    int tt = 1;

    for (int i = 1; i <= tt; ++i) {
        solve();
    }

    return 0;
}

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