Submission #1305725

#	Time	Username	Problem	Language	Result	Execution time	Memory
1305725		chithanhnguyen	Network (BOI15_net)	C++20	100 / 100	212 ms	46948 KiB

net

#include <bits/stdc++.h>
using namespace std;

#define int long long
#define ull unsigned long long
#define ld long double
#define pii pair<int, int>
#define fi first
#define se second
#define __builtin_popcount __builtin_popcountll
#define all(x) (x).begin(), (x).end()
#define BIT(x, i) (((x) >> (i)) & 1)
#define MASK(x) ((1ll << (x)))

#define debug(a, l, r) for (int _i = (l); _i <= (r); ++_i) cout << (a)[_i] << ' '; cout << '\n';

const int MAXN = 5e5 + 5;
int n, deg[MAXN], st[MAXN], timer = 0;
vector<int> adj[MAXN];
vector<int> leaves;

void dfs(int u, int par = -1) {
    st[u] = ++timer;

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

void solve() {
    cin >> n;
    for (int i = 1; i < n; ++i) {
        int u, v; cin >> u >> v;
        adj[u].push_back(v);
        adj[v].push_back(u);
        deg[u]++; deg[v]++;
    }

    dfs(1);

    for (int u = 1; u <= n; ++u) if (deg[u] == 1) leaves.push_back(u);
    sort(all(leaves), [&] (int x, int y) {
        return st[x] < st[y];
    });

    int numLeaves = (int)leaves.size();
    cout << (numLeaves + 1) / 2 << '\n';
    for (int i = 0; i < numLeaves / 2; ++i) {
        cout << leaves[i] << ' ' << leaves[i + numLeaves / 2] << '\n';
    }

    if (numLeaves & 1) cout << leaves[numLeaves - 1] << ' ' << leaves[0];
}

signed main() {
    #ifdef NCTHANH
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
    #endif
    ios_base::sync_with_stdio(0);
    cin.tie(nullptr); cout.tie(nullptr);

    solve();

    return 0;
}

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