Submission #255213

#	Time	Username	Problem	Language	Result	Execution time	Memory
255213		anubhavdhar	Network (BOI15_net)	C++14	100 / 100	538 ms	51576 KiB

net

#include<bits/stdc++.h>

#define ll long long
#define pb push_back
#define mp make_pair
#define pii pair<int, int>
#define pll pair<ll, ll>
#define ff first 
#define ss second
#define vi vector<int>
#define vl vector<ll>
#define vii vector<pii>
#define vll vector<pll>
#define FOR(i,N) for(i=0;i<(N);++i)
#define FORe(i,N) for(i=1;i<=(N);++i)
#define FORr(i,a,b) for(i=(a);i<(b);++i)
#define FORrev(i,N) for(i=(N);i>=0;--i)
#define F0R(i,N) for(int i=0;i<(N);++i)
#define F0Re(i,N) for(int i=1;i<=(N);++i)
#define F0Rr(i,a,b) for(ll i=(a);i<(b);++i)
#define F0Rrev(i,N) for(int i=(N);i>=0;--i)
#define all(v) (v).begin(),(v).end()
#define dbgLine cerr<<" LINE : "<<__LINE__<<"\n"
#define ldd long double

using namespace std;

const int Alp = 26;
const int __PRECISION = 9;
const int inf = 1e9 + 8;

const ldd PI = acos(-1);
const ldd EPS = 1e-7;

const ll MOD = 1e9 + 7;
const ll MAXN = 5e5 + 5;
const ll ROOTN = 320;
const ll LOGN = 18;
const ll INF = 1e18 + 1022;

int N;
vi leaf, g[MAXN];

void dfs(int a, int par)
{
	if(g[a].size() == 1)
		leaf.pb(a);
	for(int b : g[a])
		if(b != par)
			dfs(b, a);
}

signed main()
{

	
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	cout.tie(NULL);
	

	cin>>N;
	F0R(i, N-1)
	{
		int j, k;
		cin>>j>>k;
		g[j].pb(k);
		g[k].pb(j);
	}

	if(N == 2)
	{
		cout<<"1\n1 2\n";
		exit(0);
	}

	int leafCount = 0, r = 0;
	F0Re(i, N)
		if(g[i].size() == 1)
				++leafCount;
		else
			r = i;

	dfs(r, -1);

	if(leafCount%2)
		leaf.pb(r);

	cout<<(leafCount+1)/2<<'\n';

	F0R(i, (leafCount+1)/2)
		cout<<leaf[i]<<' '<<leaf[i + (leafCount + 1)/2]<<'\n';

	return 0;
}
/*
8
1 2
2 3
3 4
4 5
3 6
3 7
3 8
*/

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