Submission #606800

#	Time	Username	Problem	Language	Result	Execution time	Memory
606800		ThMinh_	Network (BOI15_net)	C++14	63 / 100	674 ms	262144 KiB

net

#include<bits/stdc++.h>
#define forin(i,a,b) for(int i=a;i<=b;++i)
#define forv(a,b) for(auto &a:b)
#define ii pair<int, int>
using namespace std;
const int N = 5e5+10;
int n, m, cnt, tim, root;
int low[N], num[N], color[N], vis[N], id[N];
vector<int> loop[N], ke[N];
vector<ii> adj[N], ans;
stack<int> st;
void anal(int u, int edad) {
	low[u] = num[u] = ++tim;
	vis[u] = 1; st.push(u);
	forv(e, adj[u]) {
		int v = e.first, eid = e.second;
		if(edad == eid) continue;
		if(!vis[v]) {
			anal(v, eid);
			low[u] = min(low[u], low[v]);
		}
		else if(vis[v] == 1) low[u] = min(low[u], num[v]);
	}
	if(low[u] == num[u]) {
		int v; cnt++;
		do {
			v = st.top(); st.pop();
			loop[cnt].push_back(v);
			id[v] = cnt;
			vis[v] = 2;
		} while(v != u);
	}
}
queue<int> dfs(int u) {
	color[u] = 1;
	queue<int> res;
	if(ke[u].size() == 1) {res.push(u); return res;}
	forv(v, ke[u]) if(!color[v]) {
		queue<int> q = dfs(v);
		if(res.size() + q.size() > 2 && res.size() > 0 && q.size() > 0) {
			ans.push_back({res.front(), q.front()});
			res.pop(); q.pop();
		}
		while(!q.empty()) {res.push(q.front()); q.pop();}
	}
	return res;
}
int main () {
	cin.tie(0)->sync_with_stdio(0);
	if(fopen("Task.inp","r")) {
		freopen("Task.inp","r",stdin);
		freopen("WA.out","w",stdout);
	}
	if(fopen("BlueHouse.inp","r")) {
		freopen("BlueHouse.inp","r",stdin);
		freopen("BlueHouse.out","w",stdout);
	}
	cin>>n;
	m = n - 1;
	forin(i,1,m) {
		int u, v; cin>>u>>v;
		adj[u].push_back({v, i});
		adj[v].push_back({u, i});
	}
	anal(1, 0);
	if(cnt == 2) {
		cout<<1<<"\n"<<loop[1].back()<<" "<<loop[2].back();
		return 0;
	}
	forin(i,1,cnt) {
		forv(u, loop[i]) {
			forv(v, adj[u]) if(id[v.first] != i) {
				ke[i].push_back(id[v.first]);
			}
		}	
	}
	queue<int> slt;
	forin(i,1,cnt) if(ke[i].size() > 1) {slt = dfs(i); root = i; break;}
	if(slt.size() == 2) {root = slt.front(); slt.pop();}
	if(slt.size() == 1) ans.push_back({root, slt.front()});
	cout<<ans.size()<<"\n";
	forv(v, ans) cout<<loop[v.first].back()<<" "<<loop[v.second].back()<<"\n";
}

Compilation message (stderr)

net.cpp: In function 'int main()':
net.cpp:51:10: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   51 |   freopen("Task.inp","r",stdin);
      |   ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
net.cpp:52:10: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   52 |   freopen("WA.out","w",stdout);
      |   ~~~~~~~^~~~~~~~~~~~~~~~~~~~~
net.cpp:55:10: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   55 |   freopen("BlueHouse.inp","r",stdin);
      |   ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
net.cpp:56:10: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   56 |   freopen("BlueHouse.out","w",stdout);
      |   ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~

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