Submission #1162221

#	Time	Username	Problem	Language	Result	Execution time	Memory
1162221		MPG	Network (BOI15_net)	C++20	100 / 100	210 ms	47012 KiB

net

//#pragma GCC optomize("Ofast")
#pragma GCC optimize("unroll-loops")
//#pragma GCC optimize("O3")
//#pragma GCC target("avx2")
//#pragma GCC target("sse,sse2,sse4.1,sse4.2") 



#include <bits/stdc++.h>
using namespace std;
typedef long long       ll;
#define     max_heap priority_queue<pair <ll, pair <ll, ll>>>
#define     min_heap priority_queue<pair <ll, ll>, vector<pair <ll, ll>>, greater<pair <ll, ll>>>
#define     sariE cin.tie(NULL); cout.tie(NULL); ios_base::sync_with_stdio(false);
#define     filE freopen("in.txt", "r", stdin); freopen("out1.txt", "w", stdout); 
#define     endl '\n'
#define     md(a) (a % mod + mod) % mod
#define     pb push_back
//mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
//cout << setprecision(5) << fixed << f;
//hash prime = 769


ll const maxn = 5e5 + 123;
ll const inf = 2e18;
ll const loG = 23;
ll const mod = 1e9 + 7;
//ll const mod = 998244353;
ll const sq = 500;
ll power(ll a, ll b, ll mod){if(b==0)return 1;if(b==1)return a;ll x = power(a, b / 2, mod);return (((x * x) % mod) * (b % 2 ? a : 1)) % mod;}



ll n, st[maxn], tim;
vector <ll> g[maxn], arr;
vector <pair <ll, ll>> ans;

void dfs(ll v, ll p){
    st[v] = ++tim;
    for (ll u : g[v]){
        if (u != p)
            dfs(u, v);
    }
}


bool cmp(ll v, ll u){
    return st[v] < st[u];
}



void Solve(){

cin >> n;
for (int i = 1; i < n; i++){
    ll a, b; cin >> a >> b;
    g[a].pb(b);
    g[b].pb(a);
}
for (int i = 1; i < n + 1; i++){
    if (g[i].size() == 1){
        arr.pb(i);
    }
}
dfs(1, 1);
sort(arr.begin(), arr.end(), cmp);
ll sz = arr.size();
for (int i = 0; i < sz / 2; i++){
    ans.pb({arr[i], arr[i + (sz + 1) / 2]});
}
if (sz % 2)
    ans.pb({1, arr[sz / 2]});


// for (ll x : arr)
//     cout << x << ' ';
// cout << endl;


cout << (sz + 1) / 2 << endl;
for (auto p : ans)
    cout << p.first << ' ' << p.second << endl;


}





int main(){
sariE;// filE;



int test = 1;
//cin >> test;
while (test--) Solve();
return 0;
}

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