Submission #1173405

#	Time	Username	Problem	Language	Result	Execution time	Memory
1173405		InvMOD	Network (BOI15_net)	C++20	100 / 100	216 ms	75332 KiB

net

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

#define fi first
#define se second
#define gcd __gcd
#define sz(v) (int) v.size()
#define pb push_back
#define pi pair<int,int>
#define all(v) (v).begin(), (v).end()
#define compact(v) (v).erase(unique(all(v)), (v).end())
#define FOR(i, a, b) for(int i = (a); i <= (b); i++)
#define REV(i, a, b) for(int i = (a); i >= (b); i--)
#define dbg(x) "[" #x " = " << (x) << "]"
///#define int long long

using ll = long long;
using ld = long double;
using ull = unsigned long long;

template<typename T> bool ckmx(T& a, const T& b){if(a < b) return a = b, true; return false;}
template<typename T> bool ckmn(T& a, const T& b){if(a > b) return a = b, true; return false;}

const int N = 2e5+5;
const ll MOD = 1e9+7;
const ll INF = 1e18;


void solve()
{
    int n; cin >> n;

    vector<vector<int>> adj(n + 1, vector<int>());
    for(int i = 1; i < n; i++){
        int u,v; cin >> u >> v;
        adj[u].pb(v), adj[v].pb(u);
    }

    vector<pair<int,int>> answer;
    vector<vector<int>> leaf(n + 1, vector<int>());

    function<void(int,int)> calc = [&](int x, int p) -> void{
        if(sz(adj[x]) == 1) leaf[x].push_back(x);

        for(int v : adj[x])if(v != p){
            calc(v, x);

            if(sz(leaf[x]) < sz(leaf[v])) swap(leaf[x], leaf[v]);

            // ?: greedy cover both way from x -> children in subtree
            while(sz(leaf[x]) > 1 && sz(leaf[v])){
                answer.push_back(make_pair(leaf[v].back(), leaf[x].back()));
                leaf[x].pop_back();
                leaf[v].pop_back();
            }

            while(sz(leaf[v])) leaf[x].push_back(leaf[v].back()), leaf[v].pop_back();
        }
        return;
    };

    calc(1, -1);
    while(sz(leaf[1]) > 1){
        int u = leaf[1].back(); leaf[1].pop_back();
        int v = leaf[1].back(); leaf[1].pop_back();

        answer.push_back(make_pair(u, v));
    }

    if(sz(leaf[1])){
        answer.push_back(make_pair(leaf[1].back(), answer.back().first));
    }

    cout << sz(answer) << "\n";
    for(int i = 0; i < sz(answer); i++){
        cout << answer[i].first <<" " << answer[i].second << "\n";
    }
    return;
}

signed main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    #define name "InvMOD"
    if(fopen(name".INP", "r")){
        freopen(name".INP","r",stdin);
        freopen(name".OUT","w",stdout);
    }

    int t = 1; //cin >> t;
    while(t--) solve();
    return 0;
}

Compilation message (stderr)

net.cpp: In function 'int main()':
net.cpp:89:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   89 |         freopen(name".INP","r",stdin);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
net.cpp:90:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   90 |         freopen(name".OUT","w",stdout);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~

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