Submission #1283624

#	Time	Username	Problem	Language	Result	Execution time	Memory
1283624		wedonttalkanymore	Network (BOI15_net)	C++20	100 / 100	479 ms	192200 KiB

net

#include <bits/stdc++.h>
/*
    Phia ben kia dai duong cung chi co
    Bo cat nang niu bien thoi
*/
using namespace std;
using ll = long long;

#define int long long
#define pii pair<ll, ll>
#define fi first
#define se second

const ll N = 5e5 + 5, inf = 1e18, mod = 1e9 + 7, block = 320, lim = 19;

// trinh ko co phai di chep code huyvg huhu :(
int n;
vector <int> adj[N];
set <int> s[N];
vector <pii> path;
int pos = 0;

void dfs(int u, int par) {
    if (adj[u].size() == 1 && u != pos) {
        s[u].insert(u);
        // cout << u << '\n';
        return;
    }
    vector <set <int> > child(adj[u].size());
    int sum = 0;
    for (int i = 0; i < adj[u].size(); i++) {
        int v = adj[u][i];
        if (v != par) {
            dfs(v, u);
            for (auto x : s[v]) {
                child[i].insert(x);
                sum++;
            }
            s[v].clear();
        }
    }
//    cout << "at: " << u << ' ' << sum << '\n';
//    for (int i = 0; i < child.size(); i++) {
//	 	cout << adj[u][i] << ' ' << child[i].size() << '\n';
//	}
    if (sum % 2 == 0) {
        for (int i = 0; i < child.size(); i++) {
            if (child[i].size()) {
                s[u].insert(*child[i].begin());
                child[i].erase(child[i].begin());
                break;
            }
        }
    }
    // cout << adj[u].size() << '\n';
    for (int i = 1; i < child.size(); i++) {
        // cout << adj[u][i - 1] << ' ' << adj[u][i] << ' ' << child[i].size() << '\n';
        while(child[i].size() && child[i - 1].size()) {
            int x = *child[i].begin();
            int y = *child[i - 1].begin();
            path.emplace_back(x, y);
//            cout << "allr: " << x << ' ' << y << '\n';
            child[i].erase(x);
            child[i - 1].erase(y);
        }
        for (auto x : child[i - 1]) child[i].insert(x);
        child[i - 1].clear();
    }
    for (int i = 0; i < child.size(); i++) {
        for (auto x : child[i]) {
//        	cout << "ins: " << u << ' ' << x << '\n';
        	s[u].insert(x);
		}
    }
}

signed main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    if (fopen(".inp", "r")) {
        freopen(".inp", "r", stdin);
        freopen(".out", "w", stdout);
    }
    cin >> n;
    for (int i = 1; i < n; i++) {
        int u, v;
        cin >> u >> v;
        adj[u].push_back(v);
        adj[v].push_back(u);
    }
    for (int i = 1; i <= n; i++) {
    	if (adj[i].size() == 1) {
    		pos = i;
    		break;
		}
	}
//	cout << pos << '\n';
    dfs(pos, 0);
//    cout << s[pos].size() << '\n';
//    for (auto [x, y] : path) cout << x << ' ' << y << '\n';
    if (s[pos].size()) {
        vector <int> val;
        for (auto x : s[pos]) {
        	val.push_back(x);
//        	cout << "need: " << x << '\n';
		}
        if (val.size() % 2 == 1) {
        	for (int i = 1; i < val.size(); i += 2) path.emplace_back(val[i], val[i - 1]);
        	path.emplace_back(val.back(), pos);
		}
		else {
			int sz = val.size();
			for (int i = 1; i < val.size() - 2; i += 2) path.emplace_back(val[i], val[i - 1]);
			path.emplace_back(val[sz - 1], pos);
			path.emplace_back(val[sz - 2], pos);
		}
    }
    cout << path.size() << '\n';
    for (auto [x, y] : path) cout << x << ' ' << y << '\n';
    return 0;
}

Compilation message (stderr)

net.cpp: In function 'int main()':
net.cpp:81:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   81 |         freopen(".inp", "r", stdin);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~
net.cpp:82:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   82 |         freopen(".out", "w", stdout);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~~

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