Submission #605064

#	Time	Username	Problem	Language	Result	Execution time	Memory
605064		tamthegod	Network (BOI15_net)	C++14	0 / 100	14 ms	23768 KiB

net

#include<bits/stdc++.h>

#define int long long
#define pb push_back
#define fi first
#define se second
using namespace std;
using ll = long long;
using ld = long double;
using ull = unsigned long long;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
const int maxN = 1e6 + 5;
const int mod = 1e9 + 7;
const ll oo = 1e18;
int n;
vector<int> adj[maxN];
int depth[maxN];
void ReadInput()
{
    cin >> n;
    for(int i=1; i<n; i++)
    {
        int u, v;
        cin >> u >> v;
        adj[u].pb(v);
        adj[v].pb(u);
    }
}
void dfs(int u, int par)
{
    for(int v : adj[u])
    {
        if(v == par) continue;
        depth[v] = depth[u] + 1;
        dfs(v, u);
    }
}
void Solve()
{
    dfs(1, 0);
    vector<int> leaf;
    for(int i=1; i<=n; i++)
    {
        if(adj[i].size() == 1) leaf.pb(i);
    }
    sort(leaf.begin(), leaf.end(), [](int i, int j)
    {
        return depth[i] < depth[j];
    });
    if(leaf.size() % 2 == 0)
    {
        cout << leaf.size() / 2 << '\n';
        int l = 0, r = leaf.size() - 1;
        while(l < r)
        {
            cout << leaf[l] << " " << leaf[r] << '\n';
            l++;
            r--;
        }
        return;
    }
    cout << leaf.size() / 2 + 1 << '\n';
    int l = 0, r = leaf.size() - 1;
    while(l < r)
    {
        cout << leaf[l] << " " << leaf[r] << '\n';
        l++;
        r--;
    }
    for(int i=1; i<=n; i++)
    {
        if(i != adj[leaf.back()][0])
        {
            cout << leaf.back() << " " << i;
            return;
        }
    }
}
int32_t main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    ReadInput();
    Solve();
}

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