Submission #605817

#	Time	Username	Problem	Language	Result	Execution time	Memory
605817		buidangnguyen05	Network (BOI15_net)	C++14	0 / 100	16 ms	24224 KiB

net

//source: https://oj.uz/problem/view/BOI15_net

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

typedef long long ll;

const int N = 5e5 + 10;
vector<int> adj[N]; 
vector<pair<int, int>> ans;

vector<int> dfs(int x, int par) {
	if (adj[x].size() == 1) return vector<int>(1, x);
	vector<int> cur; 
	for (int i : adj[x]) if (i != par) {
		vector<int> v = dfs(i, x);
		if (cur.size() < v.size()) swap(v, cur);
		while (cur.size() + v.size() > (!par ? 0 : 2) && v.size()) {
			int p = cur.back(); cur.pop_back();
			int q = v.back(); v.pop_back();
			ans.emplace_back(p, q);
		}
		for (int j : v) cur.push_back(j);
	}
	return cur;
}

signed main() {
	cin.tie(0)->sync_with_stdio(0);
	if (fopen("task.inp", "r")) {
		freopen("task.inp", "r", stdin);
		freopen("task.out", "w", stdout);
	}
	
	int n; cin >> n;
	for (int i = 1; i < n; ++i) {
		int x, y; cin >> x >> y;
		adj[x].push_back(y); adj[y].push_back(x);
	}

	int root = 0, save1 = 0, save2 = 0;
	for (int i = 1; i <= n; ++i) {
		if (adj[i].size() > 1) root = i;
		else if (!save1) save1 = i;
		else save2 = i;
	}
	vector<int> rem = dfs(root, 0);
	assert(rem.size() < 2);
	if (rem.size()) ans.emplace_back(rem.back(), (save1 == rem.back() ? save2 : save1));
	cout << ans.size() << "\n";
	for (pair<int, int> i : ans) cout << i.first << " " << i.second << "\n";
}

Compilation message (stderr)

net.cpp: In function 'int main()':
net.cpp:31:10: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   31 |   freopen("task.inp", "r", stdin);
      |   ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
net.cpp:32:10: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   32 |   freopen("task.out", "w", stdout);
      |   ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~

• Subtask 1: 0 / 18
• Subtask 2: 0 / 45
• Subtask 3: 0 / 37