This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include <bits/stdc++.h>
#pragma GCC optimize("O3", "unroll-loops")
#define ll long long
#define int long long
#define pb push_back
#define fi first
#define se second
#define lf (id<<1)
#define rg ((id<<1)|1)
#define md ((l+r)>>1)
#define ld long double
using namespace std;
typedef pair<int,int> pii;
typedef pair<pii, int> ipii;
const int MAXN = 3e3+10;
const int INF = 1e18+10;
const int LOG = 19;
const int MOD = 1e9+7;
const int SQRT = 450;
const vector<int> NOL = {};
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
const vector<int> dx = {1, -1, 0, 0};
const vector<int> dy = {0, 0, 1, -1};
int n;
vector <int> adj[MAXN];
vector <pii> ans;
set <pii> s;
int root, cnt, deg[MAXN];
vector <int> leaf[MAXN];
void dfs(int nw, int par){
if(adj[nw].size()==1) leaf[cnt].pb(nw);
for(auto nx : adj[nw]){
if(nx==par) continue;
dfs(nx, nw);
}
}
signed main(){
// ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
cin >> n;
for(int i=1; i<=n-1; i++){
int x, y; cin >> x >> y;
adj[x].pb(y); adj[y].pb(x);
}
int LE = 0;
for(int i=1; i<=n; i++){
if(adj[i].size()!=1) root = i;
else LE++;
}
for(auto nx : adj[root]){ ++cnt; dfs(nx, root); }
for(int i=1; i<=cnt; i++){
deg[i] = leaf[i].size(); s.insert({leaf[i].size(), i});
}
int las = 1;
// cout << root << "\n";
while(ans.size() != LE/2){
int fr = (*(s.begin())).se, en = (*(--s.end())).se;
// cout << fr << ' ' << en << " fr\n";
s.erase(s.begin());
if(fr != en) s.erase(--s.end());
int l = leaf[fr].back(); leaf[fr].pop_back(); deg[fr]--;
int r = leaf[en].back(); leaf[en].pop_back(); deg[en]--;
las = r;
ans.pb({l, r});
if(deg[fr] != 0) s.insert({deg[fr], fr});
if(fr != en)
if(deg[en] != 0) s.insert({deg[en], en});
}
if(LE%2){
int l = leaf[(*(s.begin())).se].back();
ans.pb({l, las});
}
cout << ans.size() << '\n';
for(auto in : ans) cout << in.fi << ' ' << in.se << '\n';
}
Compilation message (stderr)
net.cpp: In function 'int main()':
net.cpp:58:19: warning: comparison of integer expressions of different signedness: 'std::vector<std::pair<long long int, long long int> >::size_type' {aka 'long unsigned int'} and 'long long int' [-Wsign-compare]
58 | while(ans.size() != LE/2){
| ~~~~~~~~~~~^~~~~~~
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