Submission #656028

#	Time	Username	Problem	Language	Result	Execution time	Memory
656028		mosaev	Network (BOI15_net)	C++17	18 / 100	23 ms	35796 KiB

net

#include <bits/stdc++.h>
#include <set>
#include <map>
#include <unordered_map>
using namespace std;
#define ll long long
#define N (ll)(5e5+5)
#define NP (2097155ll)
#define M ((ll)1e9+7)
#define fi first
#define se second
ll n, ct, r;
vector<ll> v[N];
vector<pair<ll, ll>> ans;
set<pair<ll, ll>> st[N], tmp;
void dfs(ll s, ll p) {
	ct++; if (v[s].size() == 1) { st[s].insert({ ct, s }); return; }
	for (auto x : v[s]) if (x != p) {
		dfs(x, s); if (st[x].size() > st[s].size()) swap(st[x], st[s]);
		for (auto y : st[x]) st[s].insert(y); st[x].clear();
	}
	tmp.clear(); for (auto x : st[s]) tmp.insert({ -x.first, x.second });
	while (st[s].size() > 2) {
		ans.push_back({ (*st[s].begin()).second, (*tmp.begin()).second });
		st[s].erase({ -(*tmp.begin()).first, tmp.begin()->second }); tmp.erase(tmp.begin());
		tmp.erase({ -st[s].begin()->first, st[s].begin()->second }); st[s].erase(st[s].begin());
	}
}
int main() {
#define int ll
	ios_base::sync_with_stdio(false), cin.tie(0), cout.tie(0);
	cin >> n; ll a, b; for (int i = 1; i < n; i++) cin >> a >> b, v[a].push_back(b), v[b].push_back(a);
	for (int i = 1; i <= n; i++) if (v[i].size() > 1) r = i;
	dfs(r, 0);
	if (st[r].size() == 2) {
		pair<ll, ll> c = *st[r].begin(); st[r].erase(st[r].begin());
		ans.push_back({ c.second, st[r].begin()->second });
	}
	else if (st[r].size() == 1) {
		if (v[st[r].begin()->second][0] != r) ans.push_back({ r, st[r].begin()->second });
		else ans.push_back({ st[r].begin()->second, ans.back().second });
	}
	cout << ans.size() << '\n';
	for (auto x : ans) cout << x.first << ' ' << x.second << '\n';
}

Compilation message (stderr)

net.cpp: In function 'void dfs(long long int, long long int)':
net.cpp:20:3: warning: this 'for' clause does not guard... [-Wmisleading-indentation]
   20 |   for (auto y : st[x]) st[s].insert(y); st[x].clear();
      |   ^~~
net.cpp:20:41: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'for'
   20 |   for (auto y : st[x]) st[s].insert(y); st[x].clear();
      |                                         ^~

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