Submission #656266

#	Time	Username	Problem	Language	Result	Execution time	Memory
656266		ilia_rr	Network (BOI15_net)	C++17	0 / 100	32 ms	55144 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 c, n, u[N], v[N], d[N], stc, l, r, co[N], mv = 1e9;

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

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);
        }
    }
}

void dfs0(int inx, int par)
{
    d[inx] = d[par] + 1;

    co[inx] = (g[inx].size() == 1);

    for (auto i : g[inx])
    {
        if (d[i])
            continue;
        
        dfs0(i, inx);

        co[inx] += co[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[i] >> v[i], g[u[i]].push_back(v[i]), g[v[i]].push_back(u[i]);
    
    dfs0(1, 0);

    for (int i = 1; i < n; i++)
    {
        if (d[u[i]] < d[v[i]])
            swap(u[i], v[i]);

        if (abs(co[v[i]] - (co[1] - co[v[i]])) < mv)
        {
            mv = abs(co[v[i]] - (co[1] - co[v[i]]));
            
            if (co[v[i]] < (co[1] - co[v[i]]))
                c = u[i];

            else
                c = v[i];
        }
    }

    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();
        }
        
        while (l < r && 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