Submission #81563

#	Time	Username	Problem	Language	Result	Execution time	Memory
81563		arman_ferdous	Network (BOI15_net)	C++17	0 / 100	2068 ms	14404 KiB

net

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

const int N = 5e5+10;
int n, d[N];
vector<int> g[N], leaf;

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count()); // use mt19937_64 for 64 bit
int Rand(int l, int r) { return uniform_int_distribution<int>(l, r)(rng); }

int tym, st[N], low[N], vis[N];
int dfs(int u, int f) {
	vis[u] = 1;
	low[u] = st[u] = tym++;
	for(int v : g[u]) {
		if(v == f) continue;
		if(vis[v]) low[u] = min(low[u], st[v]);
		else {
			int good = dfs(v,u);
			if(!good) return 0;
			low[u] = min(low[u], low[v]);
		}
		if(d[u] < low[v]) return 0;
	} return 1;
}

bool ok() {
	memset(vis,0,sizeof vis);
	tym = 0;
	return dfs(1,-1);
}

int main() {
	scanf("%d", &n);
	for(int i = 1; i < n; i++) {
		int u, v; scanf("%d %d", &u, &v);
		g[u].push_back(v);
		g[v].push_back(u);
		d[u]++, d[v]++;
	}
	for(int i = 1; i <= n; i++) if(d[i] == 1)
		leaf.push_back(i);

	int sz = leaf.size();

	printf("%d\n", (int)ceil(sz/2.));

	for(;;) {
		for(int i = 0; i < sz; i++)
			swap(leaf[i], leaf[Rand(0,sz-1)]);

		for(int i = 0; i < sz; i+=2) {
			if(i+1 < sz) {
				g[leaf[i]].push_back(leaf[i+1]);
				g[leaf[i+1]].push_back(leaf[i]);
			} else {
				g[leaf[i]].push_back(leaf[0]);
				g[leaf[0]].push_back(leaf[i]);
			}
		}

		if(ok()) {
			for(int i = 0; i < sz; i+=2) {
				if(i+1 < sz) printf("%d %d\n", leaf[i], leaf[i+1]);
				else printf("%d %d\n", leaf[i], leaf[0]);
			} return 0;
		}

		for(int i = 0; i < sz; i+=2) {
			if(i+1 < sz) {
				g[leaf[i]].pop_back();
				g[leaf[i+1]].pop_back();
			} else {
				g[leaf[i]].pop_back();
				g[leaf[0]].pop_back();
			}
		}
	}
	return 0;	
}

Compilation message (stderr)

net.cpp: In function 'int main()':
net.cpp:34:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d", &n);
  ~~~~~^~~~~~~~~~
net.cpp:36:18: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
   int u, v; scanf("%d %d", &u, &v);
             ~~~~~^~~~~~~~~~~~~~~~~

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