Submission #1173415

#	Time	Username	Problem	Language	Result	Execution time	Memory
1173415		Minbaev	Network (BOI15_net)	C++20	100 / 100	383 ms	97428 KiB

net

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>

#pragma GCC target("avx,avx2,fma")
#pragma GCC optimize("Ofast,unroll-loops")

using namespace std;
using namespace __gnu_pbds;

#define pb 		push_back
#define all(x)	x.begin(),x.end()
#define int 	long long
#define ar 		array

template<class T>bool umax(T &a,T b){if(a<b){a=b;return true;}return false;}
template<class T>bool umin(T &a,T b){if(b<a){a=b;return true;}return false;}

template<class T> using ste = tree<T, null_type, less_equal<T>,
rb_tree_tag, tree_order_statistics_node_update>;

namespace FAST {
    template<typename T, typename F>
    istream &operator>>(istream &cin, pair<T, F> &p) {
        cin >> p.first >> p.second;
        return cin;
    }

    template<typename T, typename F>
    ostream &operator<<(ostream &cout, pair<T, F> &p) {
        cout << p.first << ' ' << p.second;
        return cout;
    }

    template<typename T>
    istream &operator>>(istream &cin, vector<T> &a) {
    for (T &i: a) cin >> i;
        return cin;
    }

    template<typename T>
    ostream &operator<<(ostream &cout, vector<T> &a) {
        for (T i: a) cout << i << ' ';
        return cout;
    }

    template<typename T>
    istream &operator>>(istream &cin, deque<T> &a) {
        for (T &i: a) cin >> i;
        return cin;
    }

    template<typename T>
    ostream &operator<<(ostream &cout, deque<T> &a) {
        for (T i: a) cout << i << ' ';
        return cout;
    }
}
using namespace FAST;

const int inf = 3e18 + 7;
const int mod = 1e9 + 7;
const int N = 5e5 + 5;
const int md = 998244353;

int binpow(int a, int b, int m){
	if(b == 0)return 1;
	if(b % 2 == 0){
		int c = binpow(a,b/2,m);
		return (c*c)%m;
	}
	return (binpow(a,b-1,m)*a)%m;
}

int divi(int a, int b, int m){
	return (a*(binpow(b,m-2, m)))%m;
}

int n,m,k,q;
vector<int> g[N], dep(N);
vector<ar<int,2>>ans;

vector<int> dfs(int x, int pr){
	if(g[x].size() == 1){
		return {x};
	}
	
	vector<int>v;
	for(auto to:g[x]){
		if(to == pr)continue;
		vector<int>vs = dfs(to, x);
//		cout << to << " " << vs << "\n";
		if(v.empty()){
			v = vs;
			continue;
		}
		
		if(v.size() < vs.size())swap(v, vs);
		
		while(v.size() > 1 && !vs.empty()){
			ans.pb({v.back(), vs.back()});
			v.pop_back();
			vs.pop_back();
		}
		for(auto to:vs)v.pb(to);
	}
	return v;
}

void solve(){
	
	cin >> n;
	for(int i = 2;i<=n;i++){
		int a,b;
		cin >> a >> b;
		g[a].pb(b);
		g[b].pb(a);
	}
	
	int a = 1;
	while(g[a].size() == 1)a += 1;
//	cout << a << "\n";
	vector<int> u = dfs(a, -1);
	for(int i = 1;i<u.size();i+=2){
		ans.pb({u[i-1], u[i]});
	}
	
	if(u.size() % 2 == 1){
		ans.pb({a, u[u.size()-1]});
	}
	
	cout << ans.size() << "\n";
	for(auto [q,w] : ans)cout << q << " " << w << "\n";
	
	
	
	
	
	
	
	
	
	
	
	
}
/*

*/
 signed main()
{
//	freopen("seq.in", "r", stdin);
//  freopen("seq.out", "w", stdout);
	ios_base::sync_with_stdio(0);cin.tie(NULL);cout.tie(NULL);
	int tt=1;//cin>>tt;
	while(tt--)solve();

}

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