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>
using namespace std;
#define in ({int x=0;int c=getchar(),n=0;for(;!isdigit(c);c=getchar()) n=(c=='-');for(;isdigit(c);c=getchar()) x=x*10+c-'0';n?-x:x;})
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int rnd(int l,int r)
{
return l+rng()%(r-l+1);
}
#define fasty ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0);
#define forinc(x,a,b) for(int x=a;x<=b;x++)
#define fordec(x,a,b) for(int x=a;x>=b;x--)
#define forv(a,b) for(auto&a:b)
#define fi first
#define se second
#define pb push_back
#define ii pair<int,int>
#define mt make_tuple
#define all(a) a.begin(),a.end()
#define reset(f,x) memset(f,x,sizeof(f))
#define getbit(x,i) ((x>>i)&1)
#define batbit(x,i) (x|(1ll<<i))
#define tatbit(x,i) (x&~(1<<i))
#define gg exit(0);
const int N = 1e5 + 100;
vector<int> ke[N];
set<int> adj[N];
int n, m;
int num[N], low[N], it, del[N];
stack<int> st;
int cnt;
int id[N], two[N];
void dfs(int u,int p)
{
num[u]=low[u]=++it;
st.push(u);
for(auto v:ke[u])
{
if(del[v]) continue;
if(v==p) continue;
if(!num[v])
{
dfs(v,u);
low[u]=min(low[u],low[v]);
}
else low[u]=min(low[u],num[v]);
}
if(num[u]==low[u])
{
cnt++;
int v;
while(1)
{
v=st.top();
st.pop();
id[v]=cnt;
del[v]=1;
two[cnt] = v;
if(v==u) break;
}
}
}
vector<int> leaf;
void dfs2(int u, int pa)
{
if(adj[u].size() == 1) leaf.pb(two[u]);
for(auto v : adj[u])
{
if(v == pa) continue;
dfs2(v,u);
}
}
main()
{
fasty;
// freopen("bluehouse.inp" , "r" ,stdin);
// freopen("bluehouse.out" , "w", stdout );
// freopen("task.inp" , "r", stdin);
cin >> n;
m=n-1;
forinc(i,1,m)
{
int u,v;
cin >> u >> v;
ke[u].pb(v);
ke[v].pb(u);
}
forinc(i,1,n) if(!num[i]) dfs(i,0);
forinc(i,1,n)
{
for(auto v : ke[i])
{
int x = id[i];
int y = id[v];
if(x!=y)
{
adj[x].insert(y);
adj[y].insert(x);
}
}
}
dfs2(1,0);
int l=0;
int step = leaf.size()/2;
if(leaf.size() % 2 == 0)
{
cout << leaf.size()/2 << "\n";
while(l + leaf.size()/2 < leaf.size())
{
cout << leaf[l] << " " << leaf[l + step] << "\n";
l++;
}
return 0;
}
leaf.pb(leaf[0]);
step++;
cout << leaf.size()/2 << "\n";
while(l + step < leaf.size())
{
cout << leaf[l] << " " << leaf[l + step] << "\n";
l++;
}
}
Compilation message (stderr)
net.cpp:75:1: warning: ISO C++ forbids declaration of 'main' with no type [-Wreturn-type]
75 | main()
| ^~~~
net.cpp: In function 'int main()':
net.cpp:122:20: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
122 | while(l + step < leaf.size())
| ~~~~~~~~~^~~~~~~~~~~~~
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