Submission #928634

#	Time	Username	Problem	Language	Result	Execution time	Memory
928634		Regulus	Network (BOI15_net)	C++17	100 / 100	457 ms	87400 KiB

net

#include <bits/stdc++.h>
#define IO ios::sync_with_stdio(false);cin.tie(0);
#define debug(x) cerr << #x << " = " << (x) << ' '
#define bug(x) cerr << (x) << ' '
#define endl cerr << '\n'
#define all(v) (v).begin(), (v).end()
#define SZ(v) (ll)(v).size()
#define lowbit(x) (x)&-(x)
#define pb emplace_back
#define F first
#define S second
using namespace std;
using ll = long long;
using pll = pair<ll, ll>;

const int N = 5e5+5;
int sz[N], cnt, pos, mxs[N];
vector<int> g[N], vec[N];
vector<pll> ans;
priority_queue<pll> pq;

inline void dfs(int x, int fa)
{
    if (SZ(g[x]) == 1) sz[x] = 1;
    for (int v : g[x])
    {
        if (v == fa) continue;
        dfs(v, x), sz[x] += sz[v];
        mxs[x] = max(mxs[x], sz[v]);
    }
    mxs[x] = max(mxs[x], cnt-sz[x]);
    if (mxs[x] < mxs[pos]) pos = x;
}

inline void dfs2(int x, int fa, int d)
{
    if (SZ(g[x]) == 1) vec[d].pb(x);
    for (int v : g[x]) if (v != fa) dfs2(v, x, d);
}

int main(void)
{ IO
    int n, i;

    cin >> n;
    for (i=0; i < n-1; ++i)
    { int x, y; cin >> x >> y, g[x].pb(y), g[y].pb(x); }

    for (i=1; i <= n; ++i) if (SZ(g[i]) == 1) ++cnt;
    pos = 0, mxs[0] = 2e9, dfs(1, 0);
    int tot = 0;
    for (int v : g[pos])
    {
        dfs2(v, pos, ++tot);
        pq.push({SZ(vec[tot]), tot});
    }

    while (SZ(pq) > 1)
    {
        auto p = pq.top(); pq.pop();
        auto p2 = pq.top(); pq.pop();
        ans.pb(vec[p.S].back(), vec[p2.S].back());
        vec[p.S].pop_back(), vec[p2.S].pop_back();
        if (!vec[p.S].empty()) pq.push({SZ(vec[p.S]), p.S});
        if (!vec[p2.S].empty()) pq.push({SZ(vec[p2.S]), p2.S});
    }
    if (!pq.empty())
    {
        int x = pq.top().S;
        while (!vec[x].empty())
            ans.pb(vec[x].back(), pos), vec[x].pop_back();
    }

    cout << SZ(ans) << '\n';
    for (auto p : ans) cout << p.F << ' ' << p.S << '\n';


    return 0;
}

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