Submission #394655

#	Time	Username	Problem	Language	Result	Execution time	Memory
394655		Hossein29	Network (BOI15_net)	C++14	100 / 100	690 ms	62916 KiB

net

#include <bits/stdc++.h>
#define int long long
#define pb push_back
using namespace std;

const int maxn = 5e5+10;
const int inf = 1e18;
const int mod = 1e9+7;

int n,cnt,root(-1);
int pendant[2][maxn],deg[maxn]; // 0 -> max , 1 -> sum
vector<int>G[maxn];
vector<int>ans;

void dfs(int v,int par){
    for(int b : G[v]){
        if(b == par) continue;
        dfs(b,v);
        pendant[1][v] += pendant[1][b];
        pendant[0][v] = max(pendant[0][v],pendant[1][b]);
    }
    if(deg[v] == 1){
        pendant[1][v]++;
    }
    pendant[0][v] = max(pendant[0][v],cnt-pendant[1][v]);
    if(pendant[0][v] <= cnt/2 && root == -1 && deg[v] != 1)
        root = v;
}

void dfss(int v,int par){
    if(deg[v] == 1)
        ans.pb(v);
    for(int b : G[v]){
        if(b == par) continue;
        dfss(b,v);
    }
}

int32_t main(){
    ios:: sync_with_stdio(0),cin.tie(0),cout.tie(0);
    ////////////////////////////////////////////////
    cin >> n;
    for(int i = 1;i<n;i++){
        int a,b; cin >> a >> b;
        G[a].pb(b),G[b].pb(a);
        deg[a]++,deg[b]++;
    }
    for(int i = 1;i<=n;i++){
        if(deg[i] == 1){
            cnt++;
        }
    }
    dfs(1,-1);
    dfss(root,-1);
    int sz = ans.size();
    if(sz&1){
        cout << (sz+1)/2 << "\n";
        int len = (sz+1)/2;
        for(int i = 0;i<(sz+1)/2;i++){
            cout << ans[i] << " " << ans[min(i+len,sz-1)] << "\n";
        }
    }
    else{
        cout << sz/2 << "\n";
        int len = sz/2;
        for(int i = 0;i<sz/2;i++){
            cout << ans[i] << " " << ans[i+len] << "\n";
        }
    }
}

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