Submission #394634

#	Time	Username	Problem	Language	Result	Execution time	Memory
394634		hivakarami	Network (BOI15_net)	C++14	100 / 100	1376 ms	91800 KiB

net

#include<bits/stdc++.h>
 
using namespace std;
 
typedef long long int ll;
typedef long double ld;
#define f first
#define s second
#define pb push_back
 
const int N = 5e5 + 100;
const ld inf = 1e8;
const ll mod = 1e9 + 7;

int r, par[N], sz[N], d[N];
vector<int> adj[N], l[N];
vector<pair<int, int>> ans;
set<pair<int, int>> st;

void dfs(int u)
{
	sz[u] = 0;
	for(auto x : adj[u])
	{
		if(x == par[u])
			continue;
		par[x] = u;
		dfs(x);
		sz[u] += sz[x];
	}
	
	if(int(adj[u].size()) == 1)
		sz[u] = 1;
}


int find(int u)
{
	//cout << u + 1 << endl;
	for(auto x : adj[u])
	{
		if(x != par[u] && sz[x] > sz[r]/2)
			return find(x);
	}
	return u;

}


void DFS(int u)
{
	for(auto x : adj[u])
	{
		if(x != par[u])
		{
			par[x] = u;
			DFS(x);
		}
	}
	
	if(adj[u].size() == 1)
		l[r].pb(u);
}



int main()
{
    
    ios_base::sync_with_stdio(false), cin.tie(0), cout.tie(0);	
	
	int n;
	cin >> n;
	for(int i = 0; i < n - 1; i++)
	{
		int x, y;
		cin >> x >> y;
		x--; y--;
		adj[x].pb(y);
		adj[y].pb(x);
	}
	
	for(int i = 0; i < n; i++)
	{
		if(int(adj[i].size()) > 1)
		{
			r = i;
			break;
		}
	}
	par[r] = r;
	dfs(r);
	
	//cout << "///////////////////////" << endl;
	
	//for(int i = 0; i < n; i++)
		//cout << i + 1 << ' ' << sz[i] << ' ' << par[i] + 1 << ' ' << adj[i].size() << endl;
	
	
	int u = find(r);
	
	//cout << u << endl;
	
	par[u] = u;
	for(auto x : adj[u])
	{
		par[x] = u;
		r = x;
		DFS(x);
		d[x] = l[x].size();
		st.insert({-d[x], x});
	}
	
	
	while(st.size() >= 2)
	{
		int x = st.begin()->s;
		st.erase(st.begin());
		int y = st.begin()->s;
		st.erase(st.begin());
		
		d[x]--; d[y]--;
		ans.pb({l[x][d[x]], l[y][d[y]]});
		
		if(d[x])
			st.insert({-d[x], x});
		
		if(d[y])
			st.insert({-d[y], y});
	}
		
	if(st.size())
	{
		int x = st.begin() -> s;
		d[x]--;
		ans.pb({u, l[x][d[x]]});		
	}	

	cout << ans.size() << endl;
	for(auto it : ans)
		cout << it.f + 1 << ' ' << it.s + 1 << endl; 

	return 0;
}


 

 

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