Submission #933687

#	Time	Username	Problem	Language	Result	Execution time	Memory
933687		oblantis	Network (BOI15_net)	C++17	0 / 100	5 ms	12376 KiB

net

#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#pragma GCC optimize("O3,unroll-loops")
#include <bits/stdc++.h>
#define all(v) v.begin(), v.end()
#define pb push_back
#define ss second
#define ff first
#define vt vector
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
const int inf = 1e9;
const int mod = 1e9+7;
const int maxn = 5e5 + 1;

vt<pii> ans;
vt<int> g[maxn];
pii dfs(int v, int p){
	int pr = -1, rp = -1;
	if(g[v].size() == 1)pr = v;
	for(auto i : g[v]){
		if(i == p)continue;
		pii wt = dfs(i, v);
		if(pr == -1)pr = wt.ff;
		else if(wt.ff != -1){
			if(rp == -1)rp = wt.ff;
			else {
				ans.pb({rp, wt.ff});
				rp = -1;
			}
		}
		if(wt.ss != -1){
			if(rp == -1)rp = wt.ss;
			else {
				ans.pb({rp, wt.ss});
				rp = -1;
			}
		}
	}
	return {pr, rp};
}
void solve() {
	int n;
	cin >> n;
	for(int i = 0, u, v; i < n - 1; i++){
		cin >> u >> v;
		g[u].pb(v), g[v].pb(u);
	}
	int t = 1;
	for(int i = 1; i <= n; i++)if((int)g[i].size() > 1)t = i;
	pii wt = dfs(t, 0);
	if(wt.ss != -1)ans.pb({wt.ff, wt.ss});
	else ans.pb({wt.ff, t});
	cout << ans.size() << '\n';
	for(auto i : ans)cout << i.ff << ' ' << i.ss << '\n';
}

int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	int times = 1;
	//cin >> times;
	for(int i = 1; i <= times; i++) {
		solve();
	}
	return 0;
}

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