Submission #828761

#	Time	Username	Problem	Language	Result	Execution time	Memory
828761		bachhoangxuan	Network (BOI15_net)	C++17	100 / 100	563 ms	252288 KiB

net

// Judges with GCC >= 12 only needs Ofast
// #pragma GCC optimize("O3,no-stack-protector,fast-math,unroll-loops,tree-vectorize")
// MLE optimization
// #pragma GCC optimize("conserve-stack")
// Old judges
// #pragma GCC target("sse4.2,popcnt,lzcnt,abm,mmx,fma,bmi,bmi2")
// New judges. Test with assert(__builtin_cpu_supports("avx2"));
// #pragma GCC target("avx2,popcnt,lzcnt,abm,bmi,bmi2,fma,tune=native")
// Atcoder
// #pragma GCC target("avx2,popcnt,lzcnt,abm,bmi,bmi2,fma")
/*
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
typedef tree<int,null_type,less<int>,rb_tree_tag,tree_order_statistics_node_update> ordered_set;
- insert(x),erase(x)
- find_by_order(k): return iterator to the k-th smallest element
- order_of_key(x): the number of elements that are strictly smaller
*/
#include<bits/stdc++.h>
using namespace std;
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
uniform_real_distribution<> pp(0.0,1.0);
//#define int long long
#define ld long double
#define pii pair<int,int>
#define piii pair<int,pii>
#define mpp make_pair
#define fi first
#define se second
//const int inf=1e18;
const int mod=998244353;
const int maxn=500005;
const int bl=650;
const int maxs=655;
const int maxm=200005;
const int maxq=1000005;
const int maxl=20;
const int maxa=1000000;
const int root=3;
/*
int power(int a,int n){
    int res=1;
    while(n){
        if(n&1) res=res*a%mod;
        a=a*a%mod;n>>=1;
    }
    return res;
}
const int iroot=power(3,mod-2);
*/
const int base=101;

pii p[maxn];
vector<pii> ans;
vector<int> edge[maxn];
void dfs(int u,int par){
    if((int)edge[u].size()==1){
        p[u]={-1,u};
        return;
    }
    deque<int> f1;
    pii f2={-1,-1};
    for(int v:edge[u]){
        if(v==par) continue;
        dfs(v,u);
        if(p[v].fi==-1) f1.push_back(p[v].se);
        else if(f2.fi==-1) f2=p[v];
        else{
            ans.push_back({f2.se,p[v].fi});
            f2.se=p[v].se;
        }
    }
    if(f2.fi!=-1){
        f1.push_front(f2.fi);
        f1.push_back(f2.se);
    }
    while((int)f1.size()>=3){
        int x=f1.back();f1.pop_back();
        int y=f1.back();f1.pop_back();
        ans.push_back({x,y});
    }
    if((int)f1.size()>=2) p[u]={f1[0],f1[1]};
    else p[u]={-1,f1[0]};
}
void solve(){
    int n;cin >> n;
    for(int i=1;i<n;i++){
        int u,v;cin >> u >> v;
        edge[u].push_back(v);
        edge[v].push_back(u);
    }
    int rt=1;
    while((int)edge[rt].size()==1) rt++;
    dfs(rt,0);
    if(p[rt].fi==-1) ans.push_back({rt,p[rt].se});
    else ans.push_back(p[rt]);
    cout << (int)ans.size() << '\n';
    for(auto &[x,y]:ans) cout << x << ' ' << y << '\n';
}

signed main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);cout.tie(NULL);
    int test=1;//cin >> test;
    while(test--) solve();
}

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