#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define fi first
#define se second
#define pb push_back
#define vi vector<int>
#define vl vector<ll>
#define pi pair<int, int>
#define pl pair<ll,ll>
#define all(x) (x).begin(),(x).end()
const int maxn=5e5+10;
vector<vi> graph(maxn);
vector<pi> ans;
pi dfs(int cur, int prev=-1) {
vi tt;
vi ttt;
int a=-1;
bool k=1;
for (auto to:graph[cur]) {
if (to==prev) {
continue;
}
pi t=dfs(to,cur);
tt.pb(t.fi);
if (t.se!=-1) {
ttt.pb(t.se);
if (k) {
a=t.fi;
}
}
else {
k=0;
a=t.fi;
}
}
if (tt.size()==1 && ttt.size()==1) {
return {tt[0],ttt[0]};
}
if (tt.size()==2 && !ttt.size()) {
return {tt[0],tt[1]};
}
if (tt.size()==0) {
return {cur,-1};
}
while (ttt.size()>1) {
int a=ttt.back();
ttt.pop_back();
int b=ttt.back();
ttt.pop_back();
ans.pb({a,b});
}
if (ttt.size()) {
ans.pb({ttt.back(),a});
tt.erase(find(all(tt),a));
}
while (tt.size()>2) {
int a=tt.back();
tt.pop_back();
int b=tt.back();
tt.pop_back();
ans.pb({a,b});
}
if (tt.size()==2) {
return {tt[0],tt[1]};
}
return {tt[0],-1};
}
int32_t main() {
ios::sync_with_stdio(0);
cin.tie(0);
int n;
cin >> n;
int a,b;
for (int i=0; i<n-1; i++) {
cin >> a >> b;
graph[--a].pb(--b);
graph[b].pb(a);
}
int i=0;
while (graph[i].size()==1) {
i++;
}
pi t=dfs(i);
if (t.se==-1) {
ans.pb({t.fi,i});
}
else {
ans.pb({t.fi,t.se});
}
cout << ans.size() << '\n';
for (int i=0; i<ans.size(); i++) {
cout << ans[i].fi+1 << ' ' << ans[i].se+1 << '\n';
}
return 0;
}
# | Verdict | Execution time | Memory | Grader output |
---|
Fetching results... |
# | Verdict | Execution time | Memory | Grader output |
---|
Fetching results... |
# | Verdict | Execution time | Memory | Grader output |
---|
Fetching results... |