Submission #252226

#	Time	Username	Problem	Language	Result	Execution time	Memory
252226		Blagojce	Network (BOI15_net)	C++11	100 / 100	978 ms	77500 KiB

net

#include <bits/stdc++.h> 
#define fr(i, n, m) for(int i = (n); i < (m); i ++)
#define pb push_back
#define st first
#define nd second
#define pq priority_queue
#define all(x) begin(x), end(x)
#include <time.h>
#include <cmath>

using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<int,int> pii;

const int i_inf = 1e9;
const ll inf = 1e18;
const ll mod = 1000000007;
const ld eps = 1e-13;
const ld pi  = 3.14159265359;
 
mt19937 _rand(time(NULL));
clock_t timer = clock();
const int mxn = 5e5;

int n;
vector<int> g[mxn];
vector<int> v;
int sz[mxn];
void dfs(int u, int p){
	sz[u] = 0;
	for(auto e : g[u]){
		if(e == p) continue;
		dfs(e, u);
		sz[u] += sz[e];
	}
	if((int)g[u].size()==1) ++sz[u];
}
int tot = 0;
int dfs1(int u, int p){
	for(auto e : g[u]){
		if(e!=p && sz[e] > tot/2) return dfs1(e, u);
	}
	return u;
}

vector<int> V;
void dfs2(int u, int p){
	for(auto e : g[u]){
		if(e == p) continue;
		dfs2(e, u);
	}
	if((int)g[u].size()==1) V.pb(u);
}


void solve(){
	cin >> n;
	fr(i, 0, n-1){
		int u, v;
		cin >> u >> v;
		
		--u, --v;
		g[u].pb(v);
		g[v].pb(u);
	}
	
	dfs(0, 0);
	tot = sz[0];
	int leaf_cen = dfs1(0, 0);
	pq<pair<int,int> > Q;
	vector<vector<int> > v;
	for(auto e : g[leaf_cen]){
		V.clear();
		dfs2(e, leaf_cen);
		Q.push({V.size(), v.size()});
		v.pb(V);
	}
	vector<pii> sol;
	
	while(!Q.empty()){
		while(!Q.empty() && (int)v[Q.top().nd].size() != (int)Q.top().st){
			Q.pop();
		}
		
		if(Q.empty())break;
		
		
		int i = Q.top().nd;
		Q.pop();
		
		while(!Q.empty() && (int)v[Q.top().nd].size() != (int)Q.top().st){
			Q.pop();
		}
		if(Q.empty())break;
		
		int j = Q.top().nd;
		Q.pop();
		
		int a = v[i].back();
		int b = v[j].back();
		
		sol.pb({a, b});
		
		v[i].pop_back();
		v[j].pop_back();
		
		
		
		if(v[i].size() > 0){
			Q.push({v[i].size(), i});
		}
		if(v[j].size() > 0){
			Q.push({v[j].size(), j});
		}
	}
	for(auto u : v){
		if(!u.empty()){
			sol.pb({u[0], leaf_cen});
		}
	}
	cout<<sol.size() << endl;
	for(auto u : sol){
		cout<<u.st+1<<' '<<u.nd+1<<endl;
	}
}

 
int main(){


	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	solve();
}

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