Submission #956696

#	Time	Username	Problem	Language	Result	Execution time	Memory
956696		GrindMachine	Network (BOI15_net)	C++17	100 / 100	506 ms	89228 KiB

net

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

template<typename T> using Tree = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
typedef long long int ll;
typedef long double ld;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;

#define fastio ios_base::sync_with_stdio(false); cin.tie(NULL)
#define pb push_back
#define endl '\n'
#define sz(a) (int)a.size()
#define setbits(x) __builtin_popcountll(x)
#define ff first
#define ss second
#define conts continue
#define ceil2(x,y) ((x+y-1)/(y))
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define yes cout << "Yes" << endl
#define no cout << "No" << endl

#define rep(i,n) for(int i = 0; i < n; ++i)
#define rep1(i,n) for(int i = 1; i <= n; ++i)
#define rev(i,s,e) for(int i = s; i >= e; --i)
#define trav(i,a) for(auto &i : a)

template<typename T>
void amin(T &a, T b) {
    a = min(a,b);
}

template<typename T>
void amax(T &a, T b) {
    a = max(a,b);
}

#ifdef LOCAL
#include "debug.h"
#else
#define debug(x) 42
#endif

/*



*/

const int MOD = 1e9 + 7;
const int N = 5e5 + 5;
const int inf1 = int(1e9) + 5;
const ll inf2 = ll(1e18) + 5;

vector<ll> adj[N];
vector<ll> sub_leaves(N);

void dfs1(ll u, ll p){
    if(sz(adj[u]) == 1){
        sub_leaves[u] = 1;
    }

    trav(v,adj[u]){
        if(v == p) conts;
        dfs1(v,u);
        sub_leaves[u] += sub_leaves[v];
    }
}

ll dfs2(ll u, ll p, ll tot){
    trav(v,adj[u]){
        if(v == p) conts;
        if(sub_leaves[v] > tot/2){
            return dfs2(v,u,tot);
        }
    }

    return u;
}

vector<ll> leaves[N];

void dfs3(ll u, ll p, ll r){
    if(sz(adj[u]) == 1){
        leaves[r].pb(u);
    }

    trav(v,adj[u]){
        if(v == p) conts;
        dfs3(v,u,r);
    }
}

void solve(int test_case)
{
    ll n; cin >> n;
    rep1(i,n-1){
        ll u,v; cin >> u >> v;
        adj[u].pb(v), adj[v].pb(u);
    }

    ll root = -1;
    rep1(i,n){
        if(sz(adj[i]) >= 2){
            root = i;
            break;
        }
    }

    assert(root != -1);
    dfs1(root,-1);
    ll c = dfs2(root,-1,sub_leaves[root]);
    trav(v,adj[c]){
        dfs3(v,c,v);
    }

    vector<pll> ans;
    priority_queue<pll> pq;
    trav(v,adj[c]){
        pq.push({sz(leaves[v]),v});
    }

    while(sz(pq) >= 2){
        auto [s1,u] = pq.top();
        pq.pop();
        auto [s2,v] = pq.top();
        pq.pop();

        ll x = leaves[u].back();
        leaves[u].pop_back();
        ll y = leaves[v].back();
        leaves[v].pop_back();

        ans.pb({x,y});

        if(s1-1 >= 1){
            pq.push({s1-1,u});
        }
        if(s2-1 >= 1){
            pq.push({s2-1,v});
        }
    }

    if(!pq.empty()){
        auto [s1,u] = pq.top();
        pq.pop();

        assert(s1 == 1);
        ll x = leaves[u].back();
        leaves[u].pop_back();
        ans.pb({c,x});
    }

    assert(pq.empty());
    
    cout << sz(ans) << endl;
    for(auto [u,v] : ans){
        cout << u << " " << v << endl;
    }
}

int main()
{
    fastio;

    int t = 1;
    // cin >> t;

    rep1(i, t) {
        solve(i);
    }

    return 0;
}

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