Submission #286621

#	Time	Username	Problem	Language	Result	Execution time	Memory
286621		Marlov	Network (BOI15_net)	C++14	0 / 100	2 ms	1792 KiB

net

/*
Code by @marlov       
*/
#include <iostream>
#include <fstream>
#include <string>
#include <sstream>
#include <vector>
#include <string>
#include <cmath>
#include <algorithm>
#include <iomanip>
#include <utility>
#include <set>
#include <unordered_set>
#include <map>
#include <unordered_map>
#include <stack>
#include <queue>
#include <iterator>
using namespace std;
typedef long long ll;
typedef pair<int,int> pi;

#define maxN 2005

int N;
int tl=0;
int leaves[maxN];
int root=0;
vector<int> adj[maxN];
vector< pair<int,int> > edges;
queue<int> vals[maxN];

void findRoot(int n,int p){
	if(adj[n].size()==1){
		leaves[n]++;
	}
	bool wks=true;
	for(int i:adj[n]) if(i!=p){
		findRoot(i,n);
		if(leaves[i]>floor(tl/2)) wks=false; 
		leaves[n]+=leaves[i];
	}
	if(tl-leaves[n]>floor(tl/2)) wks=false;
	if(wks&&adj[n].size()>1) root=n;
}

void dfs(int n,int p){
	if(adj[n].size()==1){
		vals[n].push(n);
		return;
	}
	int lv=n;
	for(int i:adj[n]) if(i!=p){
		dfs(i,n);
		if(vals[i].size()>vals[lv].size()) lv=i;
	}
	//swap(vals[n],vals[lv]);
	for(int i:adj[n]) if(i!=p){
		while(vals[n].size()>(n==root?0:1)&&vals[i].size()>0){
			edges.push_back(make_pair(vals[n].front(),vals[i].front()));
			vals[n].pop();
			vals[i].pop();
		}
		while(vals[i].size()>0){
			vals[n].push(vals[i].front());
			vals[i].pop();
		}
	}
}

int main() {
	ios_base::sync_with_stdio(0); cin.tie(0);
	cin>>N;
	int u,v;
	for(int i=0;i<N-1;i++){
		cin>>u>>v;
		u--;v--;
		adj[u].push_back(v);
		adj[v].push_back(u);
	}
	for(int i=0;i<N;i++){
		if(adj[i].size()==1) tl++;
	}
	//cout<<root<<endl;
	findRoot(0,-1);
	//cout<<root<<'\n';
	dfs(root,-1);

	while(vals[root].size()>1){
		int f=vals[root].front();vals[root].pop();
		int s=vals[root].front();vals[root].pop();
		edges.push_back(make_pair(f,s));
	}
	if(vals[root].size()==1){
		edges.push_back(make_pair(vals[root].front(),root));
	}

	cout<<edges.size()<<'\n';
	for(int i=0;i<(int)edges.size();i++){
		cout<<edges[i].first+1<<" "<<edges[i].second+1<<'\n';
	}

    return 0;
}

/* stuff you should look for
	* int overflow, array bounds
	* special cases (n=1,n=0?)
	* do smth instead of nothing and stay organized
*/

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