Submission #1225209

#	Time	Username	Problem	Language	Result	Execution time	Memory
1225209		jellybean	Network (BOI15_net)	C++20	100 / 100	198 ms	44564 KiB

net

#include <bits/stdc++.h>
using namespace std;
#define int long long
#define pb push_back
#define dd(x) cout<<#x<<" is "<<x<<endl;
#define dd2(x,y) cout<<#x<<" is "<<x<<" "<<#y<<" is "<<y<<endl;
#define dl(x) cout<<#x<<" is "<<endl; for(auto i:x) cout<<i<<' '; cout<<endl;
#define fi first
#define se second
typedef pair<int,int> pii;

const int N = 5e5+5;
vector<int>adj[N];
int st[N];

int cnt = 1;
void dfs(int x, int p){
	st[x] = cnt++;
	for(auto i: adj[x]){
		if(i == p) continue;
		dfs(i,x);
	}
}

bool cmp(int a, int b){
	return st[a] < st[b];
}

signed main(){
	ios::sync_with_stdio(0); cin.tie(0);
	
	int n; cin>>n;
	for(int i=0; i<n-1; i++){
		int a,b; cin>>a>>b;
		adj[a].pb(b);
		adj[b].pb(a);
	}
	
	int d = 0, root = -1;
	vector<int>leaves;
	
	for(int i=1; i<=n; i++) {
		if(adj[i].size() == 1) d++, leaves.pb(i);
		else root = i;
	}
	
	dfs(root,-1);
	sort(leaves.begin(), leaves.end(), cmp);
	
	cout<<(d+1)/2<<'\n';
	for(int i=0; i<(d+1)/2; i++){
		cout<<leaves[i]<<' '<<leaves[i+(d/2)]<<'\n';
	}
	
	return 0;
}

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