Submission #1008744

#	Time	Username	Problem	Language	Result	Execution time	Memory
1008744		ByeWorld	Network (BOI15_net)	C++14	100 / 100	394 ms	55892 KiB

net

#include <bits/stdc++.h>
#pragma GCC optimize("O3", "unroll-loops")
#define ll long long
#define int long long
#define pb push_back
#define fi first
#define se second
#define lf (id<<1)
#define rg ((id<<1)|1)
#define md ((l+r)>>1)
#define ld long double
using namespace std;
typedef pair<int,int> pii;
typedef pair<pii, int> ipii;
const int MAXN = 6e5+10;
const int INF = 1e18+10;
const int LOG = 19;
const int MOD = 1e9+7;
const int SQRT = 450;
const vector<int> NOL = {};
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
const vector<int> dx = {1, -1, 0, 0};
const vector<int> dy = {0, 0, 1, -1};

int n;
vector <int> adj[MAXN];
vector <pii> ans;
vector <int> leaf;
int root;

void dfs(int nw, int par){
	if(adj[nw].size()==1) leaf.pb(nw);
	for(auto nx : adj[nw]){
		if(nx==par) continue;
		dfs(nx, nw);
	}
}
signed main(){
	// ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
	cin >> n;
	for(int i=1; i<=n-1; i++){
		int x, y; cin >> x >> y;
		adj[x].pb(y); adj[y].pb(x);
	}
	for(int i=1; i<=n; i++){
		if(adj[i].size()!=1) root = i;
	}

	dfs(root, 0);
	int siz = leaf.size();
	cout << (siz+1)/2 << '\n';
	for(int i=0; i<(siz+1)/2; i++) 
		cout << leaf[i] << ' ' << leaf[i+siz/2] << '\n';
}

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