//Never stop trying
/*#pragma GCC target ("avx2")
#pragma GCC optimize ("Ofast")
#pragma GCC optimization ("O3")
#pragma GCC optimization ("unroll-loops")*/
#include "bits/stdc++.h"
using namespace std;
#define boost ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0)
typedef long long ll;
#define int ll
typedef string str;
typedef double db;
typedef long double ld;
typedef pair<int, int> pi;
#define fi first
#define se second
typedef vector<int> vi;
typedef vector<pi> vpi;
typedef vector<str> vs;
typedef vector<ld> vd;
#define pb push_back
#define eb emplace_back
#define sz(x) (int)x.size()
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
#define endl "\n"
#define FOR(i,a,b) for (int i = (a); i < (b); ++i)
#define ROF(i,a,b) for (int i = (b)-1; i >= (a); --i)
const int MOD = 1e9 + 7; //998244353
const ll INF = 1e18;
const int MX = 2e5 + 10;
const int nx[4] = {0, 0, 1, -1}, ny[4] = {1, -1, 0, 0}; //right left down up
template<class T> using V = vector<T>;
template<class T> bool ckmin(T& a, const T& b) { return a > b ? a = b, 1 : 0; }
template<class T> bool ckmax(T& a, const T& b) { return a < b ? a = b, 1 : 0; }
ll cdiv(ll a, ll b) { return a / b + ((a ^ b) > 0 && a % b); } // divide a by b rounded up
//constexpr int log2(int x) { return 31 - __builtin_clz(x); } // floor(log2(x))
mt19937 rng(chrono::system_clock::now().time_since_epoch().count());
//mt19937_64 rng(chrono::system_clock::now().time_since_epoch().count());
ll random(ll a, ll b){
return a + rng() % (b - a + 1);
}
#ifndef LOCAL
#define cerr if(false) cerr
#endif
#define dbg(x) cerr << #x << " : " << x << endl;
#define dbgs(x,y) cerr << #x << " : " << x << " / " << #y << " : " << y << endl;
#define dbgv(v) cerr << #v << " : " << "[ "; for(auto it : v) cerr << it << ' '; cerr << ']' << endl;
#define here() cerr << "here" << endl;
void IO() {
#ifdef LOCAL
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#endif
}
int N;
vpi adj[MX];
bool finished(){
FOR(u,1,N+1) for(auto ed: adj[u]) if(ed.se==1) return 0;
return 1;
}
int val[MX],mxVal[MX];
void dfs(int u, int p){
val[u]=0,mxVal[u]=0;
vi vec;
for(auto v: adj[u]) if(v.fi!=p){
dfs(v.fi,u);
int x=mxVal[v.fi]+v.se;
vec.pb(x);
ckmax(mxVal[u],x);
}
sort(rall(vec));
if(sz(vec)) val[u]=vec[0];
if(sz(vec)>1) val[u]+=vec[1];
}
int32_t main() {
boost; IO();
cin>>N;
FOR(i,0,N-1){
int u,v; cin>>u>>v;
adj[u].pb({v,1});
adj[v].pb({u,1});
}
vpi res;
while(!finished()){
dfs(1,1);
int mx=-1,u;
FOR(i,1,N+1) if(ckmax(mx,val[i])) u=i;
assert(mx>0);
//cout << u << endl;
//FOR(i,1,N+1) cout << val[i] << ' ' << mxVal[i] << endl;
int a=u,b=u,pa=-1,pb=-1;
bool taken[N+1]; fill(taken,taken+N+1,0);
while(sz(adj[a])>1 || a==1){
int mx=-1,nxt;
for(auto v: adj[a]) if(v.fi!=pa && ckmax(mx,mxVal[v.fi]+v.se)) nxt=v.fi;
for(auto &v: adj[a]) if(v.fi==nxt) v.se=0;
for(auto &v: adj[nxt]) if(v.fi==a) v.se=0;
taken[nxt]=true;
pa=a; a=nxt;
}
//for(auto v: adj[1]) cout << v.se << endl;
dfs(1,1);
/*cout << endl;
FOR(i,1,N+1) cout << val[i] << ' ' << mxVal[i] << endl;*/
while(sz(adj[b])>1 || b==1){
int mx=-1,nxt;
for(auto v: adj[b])
if(v.fi!=pb && !taken[v.fi] && ckmax(mx,mxVal[v.fi]+v.se))
nxt=v.fi;
if(mx==-1) break;
for(auto &v: adj[b]) if(v.fi==nxt) v.se=0;
for(auto &v: adj[nxt]) if(v.fi==b) v.se=0;
pb=b; b=nxt;
}
assert(a!=b);
res.pb({a,b});
/*dbg(u);
dbgs(a,b);*/
}
assert(sz(res)<N);
cout << sz(res) << endl;
for(auto x: res) cout << x.fi << ' ' << x.se << endl;
return 0;
}
/* Careful!!!
.Array bounds
.Infinite loops
.Uninitialized variables / empty containers
.Multisets are shit
Some insights:
.Binary search
.Graph representation
.Write brute force code
.Change your approach
*/
Compilation message
net.cpp: In function 'int32_t main()':
net.cpp:130:7: warning: this 'for' clause does not guard... [-Wmisleading-indentation]
130 | for(auto v: adj[b])
| ^~~
net.cpp:133:4: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'for'
133 | if(mx==-1) break;
| ^~
net.cpp:117:28: warning: 'nxt' may be used uninitialized in this function [-Wmaybe-uninitialized]
117 | for(auto &v: adj[a]) if(v.fi==nxt) v.se=0;
| ^~
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Correct |
4 ms |
5100 KB |
Output is correct |
| 2 |
Incorrect |
4 ms |
5100 KB |
Invalid number of links. |
| 3 |
Halted |
0 ms |
0 KB |
- |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Correct |
4 ms |
5100 KB |
Output is correct |
| 2 |
Incorrect |
4 ms |
5100 KB |
Invalid number of links. |
| 3 |
Halted |
0 ms |
0 KB |
- |
| # |
Verdict |
Execution time |
Memory |
Grader output |
| 1 |
Correct |
4 ms |
5100 KB |
Output is correct |
| 2 |
Incorrect |
4 ms |
5100 KB |
Invalid number of links. |
| 3 |
Halted |
0 ms |
0 KB |
- |
