| # | Time | Username | Problem | Language | Result | Execution time | Memory |
|---|---|---|---|---|---|---|---|
| 1250847 | tradz | Network (BOI15_net) | C++20 | 0 ms | 0 KiB |
#include <bits/stdc++.h>
#define TIME (1.0 * clock() / CLOCKS_PER_SEC)
#define For(i,a,b) for(int i = a; i <= b; i++)
#define Ford(i,a,b) for(int i = a; i >= b; i--)
#define ll long long
#define ii pair<int,int>
#define fi first
#define se second
#define all(v) v.begin(),v.end()
#define RRH(v) v.resize(unique(all(v)) - v.begin())
using namespace std;
const int N = 1e6+7;
const int M = 1e9+7;
const ll oo = 3e18;
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
long long GetRandom(long long l, long long r) {
return uniform_int_distribution<long long> (l, r)(rng);
}
int n, cnt = 0;
vector <int> g[N], ver[N];
int dp[N];
void dfs(int u, int rem, int pre = 0, int num) {
dp[u] = 1;
for (int v: g[u]) {
if (v == pre) continue;
if (rem == 1) {
cnt++;
dfs(v, 0, u, cnt);
}
else {
dfs(v, 0, u, num);
}
dp[u] += dp[v];
}
if (dp[u] == 1) ver[num].push_back(u);
}
int main() {
ios::sync_with_stdio(0); cin.tie(0);
#define TASK ""
if (fopen (".inp", "r")) {
freopen (".inp", "r", stdin);
freopen (".out", "w", stdout);
}
if(fopen(TASK".inp", "r")) {
freopen(TASK".inp", "r", stdin);
freopen(TASK".out", "w", stdout);
}
cin >> n;
for (int i = 1; i < n; i++) {
int u, v; cin >> u >> v;
g[u].push_back(v);
g[v].push_back(u);
}
vector <pair <int, int>> ans;
for (int i = 1; i <= n; i++) {
if (g[i].size() >= 2) {
dfs(i, 1);
break;
}
}
int i = 1;
int j = cnt;
while (i < j) {
while (ver[i].size() && ver[j].size()) {
ans.push_back({ver[i].back(), ver[j].back()});
ver[i].pop_back();
ver[j].pop_back();
}
if (ver[i].size() == 0) i++;
else j--;
}
if (i == j) {
if (ver[i].size() != 0) {
int l = 0;
int r = ver[i].size() - 1;
while (l < r) {
ans.push_back({ver[i][l], ver[i][r]});
l++;
r--;
}
if (l == r) {
ans.push_back({ans[0].fi, ver[i][l]});
}
}
}
cout << ans.size() << '\n';
for (int i = 0; i < ans.size(); i++) cout << ans[i].first << " " << ans[i].second << '\n';
cerr << "Time elapsed: " << TIME << " s.\n";
return 0;
}