Submission #75930

#	Time	Username	Problem	Language	Result	Execution time	Memory
75930		Vasiljko	Network (BOI15_net)	C++14	0 / 100	43 ms	49500 KiB

net

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
const ll MOD = 1e9+7;
const int N = 5e5+5;
const int LOG = 19;

int n,par[N][LOG],d[N];
vector<int>v[N],leaves;
vector<pair<int,int> >ans;

void dfs(int s,int p){
    if(v[s].size()==1)leaves.push_back(s);
    par[s][0]=p;
    if(p!=-1)d[s]=d[p]+1;

    for(int i=1;i<LOG;i++)if(par[s][i-1]!=-1)par[s][i]=par[par[s][i-1]][i-1];
    for(auto e:v[s])if(e!=p)dfs(e,s);
}

int LCA(int s,int t){
    if(d[s]<d[t])swap(s,t);
    int dist=d[s]-d[t];
    for(int i=LOG-1;i>=0;i--)if(dist&(1<<i))s=par[s][i];

    if(s==t)return s;

    for(int i=LOG-1;i>=0;i--){
        if(par[s][i]!=par[t][i]){
            s=par[s][i];
            t=par[t][i];
        }
    }
    return par[s][0];
}

int main()
{
    ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
    memset(par,-1,sizeof(par));
    int x,y;

    cin>>n;
    for(int i=1;i<=n-1;i++){
        cin>>x>>y;
        v[x].push_back(y);
        v[y].push_back(x);
    }

    dfs(1,-1);

    bool ok=false;
    for(int i=0;i<(int)leaves.size()/2;i++){
        if(i<leaves.size()-i-1){
            ans.push_back({leaves[i],leaves[(int)leaves.size()-i-1]});
            if(LCA(leaves[i],leaves[(int)leaves.size()-i-1])<=1)ok=true;
        }
    }
    if((int)leaves.size()%2==1){
        ans.push_back({1,leaves[(int)leaves.size()/2]});
    }else{
        if(!ok){
            if(leaves[0]!=1)ans.push_back({1,leaves[0]});
            else ans.push_back({1,leaves[1]});
        }
    }

    cout<<(int)ans.size()<<"\n";
    for(auto e:ans)cout<<e.first<<" "<<e.second<<"\n";
    return 0;
}

Compilation message (stderr)

net.cpp: In function 'int main()':
net.cpp:55:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         if(i<leaves.size()-i-1){
            ~^~~~~~~~~~~~~~~~~~

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