Submission #656255

#	Time	Username	Problem	Language	Result	Execution time	Memory
656255		ilia_rr	Network (BOI15_net)	C++17	0 / 100	31 ms	55108 KiB

net

// بِسْمِ اللَّهِ الرَّحْمَنِ الرَّحِيمِ
// Written by Ilia Rahbar

// #pragma GCC optimize ("O3,no-stack-protector,unroll-loops,fast-math")
// #pragma GCC target ("abm,bmi,bmi2,tbm,avx2")

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

#define int int64_t
#define ai(x) array <int, x>
#define F first 
#define S second

const int MOD = 1e9 + 7, N = 1e6 + 100;

int q, c, n, u, v, d[N], stc, l, r;

vector <int> g[N], st[N], ans[2];

int ffv(int inx)
{
    memset(d, 0, sizeof d);

    queue <int> q;

    d[inx] = 1;
    q.push(inx);

    while (q.size())
    {
        inx = q.front();
        q.pop();

        for (auto i : g[inx])
        {
            if (!d[i])
            {
                d[i] = d[inx] + 1;

                q.push(i);
            }
        }
    }
    
    return inx;
}

void dfs(int inx)
{
    if (g[inx].size() == 1)
        st[stc].push_back(inx);

    for (auto i : g[inx])
    {
        if (!d[i])
        {
            d[i] = 1;

            dfs(i);
        }
    }
}

int32_t main()
{
    ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    
    cin >> n;

    for (int i = 1; i < n; i++)
        cin >> u >> v, g[u].push_back(v), g[v].push_back(u);

    q = ffv(1);

    q = ffv(q);

    for (int i = 1; i <= n; i++)
    {
        if (d[i] == (d[q] + 1) / 2)
        {
            c = i;
            
            break;
        }
    }

    memset(d, 0, sizeof d);

    d[c] = 1;

    for (auto i : g[c])
    {
        dfs(i);

        stc++;
    }

    sort(st, st+stc, [](vector <int> &x, vector <int> &y){return (x.size() < y.size());});

    l = 0;
    r = stc-1;

    while (st[r].size())
    {
        for (int i = r-1; st[r].size() && i >= l; i--)
        {
            ans[0].push_back(st[r].back());
            ans[1].push_back(st[i].back());

            st[r].pop_back();
            st[i].pop_back();
        }
        
        if (st[r].empty())
            r--;

        while (l < r && st[l].empty())
            l++;

        if (l == r)
        {
            while (st[r].size())
            {
                ans[0].push_back(st[r].back());
                
                st[r].pop_back();
                
                if (st[r].size())
                {
                    ans[1].push_back(st[r].back());
                    st[r].pop_back();
                }

                else
                    ans[1].push_back(c);
            }
        }
    }

    cout << ans[0].size() << "\n";

    for (int i = 0; i < int(ans[0].size()); i++)
        cout << ans[0][i] << " " << ans[1][i] << "\n";
}

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