Submission #928630

#	Time	Username	Problem	Language	Result	Execution time	Memory
928630		Regulus	Network (BOI15_net)	C++17	0 / 100	1 ms	2972 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 = 1e5+5;
int sz[N], cnt, pos, mxs[N];
vector<int> g[N], vec, pre;
vector<pll> ans;

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

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);
    //debug(pos), endl;
    for (int v : g[pos])
    {
        dfs2(v, pos);
        while (!pre.empty() && !vec.empty())
        {
            ans.pb(pre.back(), vec.back());
            pre.pop_back(); vec.pop_back();
        }
        while (!vec.empty()) pre.pb(vec.back()), vec.pop_back();
    }
    for (int x : pre) ans.pb(x, pos);

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


    return 0;
}

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