Submission #926433

#	Time	Username	Problem	Language	Result	Execution time	Memory
926433		Regulus	Network (BOI15_net)	C++17	0 / 100	3 ms	12892 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 dis[N];
vector<int> g[N], vec;
vector<pll> v2;

inline void dfs(int x, int pre)
{
    dis[x] = dis[pre] + 1;
    for (int v : g[x]) if (v != pre) dfs(v, x);
}

inline bool cmp(int x, int y) { return dis[x] < dis[y]; }

int main(void)
{ IO
    int n, i, pos;
    
    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) vec.pb(i);
    dfs(1, 0);
    for (i=2, pos=1; i <= n; ++i) if (dis[i] > dis[pos]) pos = i;

    dfs(pos, 0);
    for (i=2, pos=1; i <= n; ++i) if (dis[i] > dis[pos]) pos = i;
    dfs(pos, 0);

    sort(all(vec), cmp);
    v2.pb(vec[0], vec.back());
    for (int i=2; i < SZ(vec)-1; i+=2) v2.pb(vec[i-1], vec[i]);
    if (SZ(vec)&1) v2.pb(vec[SZ(vec)-2], vec.back());

    cout << SZ(v2) << '\n';
    for (auto p : v2) cout << p.F << ' ' << p.S << '\n';


    return 0;
}

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