Submission #905939

#	Time	Username	Problem	Language	Result	Execution time	Memory
905939		dsyz	Network (BOI15_net)	C++17	0 / 100	4 ms	12632 KiB

net

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define MAXN (500005)
ll N, connectto1 = -1, cnt = 0;
ll pre[MAXN];
vector<ll> v[MAXN];
bool leaf[MAXN];
void dfs(ll x,ll p){
	pre[x] = cnt++;
	ll children = 0;
	ll leafchildren = 0;
	for(auto u : v[x]){
		if(u != p){
			dfs(u,x);
			leafchildren += leaf[u];
			if(leafchildren >= 2) connectto1 = u;
			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);
	vector<pair<ll,ll> > ans;
	for(ll i = 0;i < N;i++){
		if(leaf[i] && i != connectto1){
			ans.push_back({pre[i],i});
		}
	}
	sort(ans.begin(),ans.end());
	if(ans.size() + 1 == 1){ //graph is a line
		cout<<1<<'\n';
		cout<<1<<" "<<ans[0].second + 1<<'\n';
	}else if(ans.size() + 1 == 2){
		cout<<2<<'\n';
		cout<<1<<" "<<ans[0].second + 1<<'\n';
		cout<<ans[0].second + 1<<" "<<ans[1].second + 1<<'\n';
	}else{
		cout<<ans.size() - 1 + 1<<'\n';
		cout<<1<<" "<<connectto1 + 1<<'\n';
		for(ll i = 1;i < ans.size();i++){
			cout<<ans[i - 1].second + 1<<" "<<ans[i].second + 1<<'\n';
		}
	}
}

Compilation message (stderr)

net.cpp: In function 'int main()':
net.cpp:53:18: warning: comparison of integer expressions of different signedness: 'll' {aka 'long long int'} and 'std::vector<std::pair<long long int, long long int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   53 |   for(ll i = 1;i < ans.size();i++){
      |                ~~^~~~~~~~~~~~

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