Submission #906224

#	Time	Username	Problem	Language	Result	Execution time	Memory
906224		dsyz	Network (BOI15_net)	C++17	100 / 100	496 ms	54456 KiB

net

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define MAXN (500005)
ll N, cnt = 0;
ll pre[MAXN];
vector<ll> v[MAXN];
bool leaf[MAXN];
void dfs(ll x,ll p){
	pre[x] = cnt++;
	ll children = 0;
	for(auto u : v[x]){
		if(u != p){
			dfs(u,x);
			children++;
		}
	}
	if(children == 0){
		leaf[x] = 1;
	}
}
int main() {
	ios_base::sync_with_stdio(false);cin.tie(0);
	cin>>N;
	for(ll i = 0;i < N - 1;i++){
		ll a,b;
		cin>>a>>b;
		a--, b--;
		v[a].push_back(b);
		v[b].push_back(a);
	}
	dfs(0,-1);
	deque<pair<ll,ll> > ans;
	for(ll i = 0;i < N;i++){
		if(i == 0 && v[0].size() == 1){ //technically the root node can also be a "leaf" if rooted differently
			ans.push_back({pre[i],i});
		}else if(leaf[i]){
			ans.push_back({pre[i],i});
		}
	}
	sort(ans.begin(),ans.end());
	cout<<(ans.size() + 1) / 2<<'\n';
	for(ll i = 0;i < ll((ans.size() + 1) / 2);i++){ //greedily optimal to pair front and middle (intution: at least one of the pair will have lca = root node)
		cout<<ans[i].second + 1<<" "<<ans[i + (ans.size() / 2)].second + 1<<'\n';
	}
}

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