Submission #633978

#	Time	Username	Problem	Language	Result	Execution time	Memory
633978		radal	Network (BOI15_net)	C++17	100 / 100	466 ms	37132 KiB

net

#include <bits/stdc++.h>
#pragma GCC target("sse,sse2")
#pragma GCC optimize("unroll-loops,O3")
#define rep(i,l,r) for (int i = l; i < r; i++)
#define repr(i,r,l) for (int i = r; i >= l; i--)
#define X first
#define Y second
#define all(x) (x).begin() , (x).end()
#define pb push_back
#define endl '\n'
#define debug(x) cerr << #x << " : " << x << endl;
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<int,int> pll;
constexpr int N = 5e5+20,mod = 1e9+7,inf = 1e9+10;
inline int mkay(int a,int b){
    if (a+b >= mod) return a+b-mod;
    if (a+b < 0) return a+b+mod;
    return a+b;
}
 
inline int poww(int a,int k){
    if (k < 0) return 0;
    int z = 1;
    while (k){
        if (k&1) z = 1ll*z*a%mod;
        a = 1ll*a*a%mod;
        k /= 2;
    } 
    return z; 
}

vector<int> adj[N],ve;
int tin[N],T;

bool cmp(int u,int v){
    return (tin[u] < tin[v]);
}

void dfs(int v,int p){
    tin[v] = T++;
    for(int u : adj[v]) if (u != p) dfs(u,v);
}
int main(){
    ios_base :: sync_with_stdio(0); cin.tie(0);
    int n,ans = 0;
    cin >> n;
    rep(i,1,n){
        int u,v;
        cin >> u >> v;
        adj[u].pb(v);
        adj[v].pb(u);
    }
    dfs(1,0);
    rep(i,1,n+1) if (adj[i].size() == 1){
        ans++;
        ve.pb(i);
    }
    int t = (ans+1)/2;
    cout << t << endl;
    sort(all(ve),cmp);
    rep(i,0,ans){
        if (i >= t) break;
        if (i+t == ans){
            cout << ve[i] << ' ' << ve[0] << endl;
            break;
        }
        cout << ve[i] << ' ' << ve[i+t] << endl;
    }
}

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