Fragile network

#include<bits/stdc++.h>
//#include<unordered_set>
// #pragma GCC optimize("Ofast")
using namespace std;
#define int long long
#define die(x) return cout << x << endl, 0
#define FI first
#define SE second
#define all(o) o.begin(), o.end()
#define endl '\n'
#define IOS ios::sync_with_stdio(0), cin.tie(0)
#define FILE freopen("input.txt", "r", stdin), freopen("output.txt", "w", stdout)
#define SZ(x) ((int)(x).size())
#define PB push_back
#define PF push_front
#define POB pop_back
#define POF pop_front
#define MP make_pair
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef map<int,int> mpi;
typedef set<int> sti;
typedef vector<pii> vii;
typedef map<pii,int> mpii;
typedef set<pii> stii;
typedef long double ld;
typedef long long ll;
int gcd(int x,int y){ return (!y ? x : gcd(y, x%y)); }
int power(int x, int y) { return (!y ? 1 : power(x, y / 2) * power(x, y / 2) * (y % 2 ? x : 1)); }
int to_int(string sconvert){stringstream geek(sconvert);int xconvert = 0; geek >> xconvert; return xconvert;}
int fastMax(int x, int y) { return (((y-x)>>(32-1))&(x^y))^y; }
int fastMin(int x, int y) { return (((y-x)>>(32-1))&(x^y))^x; }
const int MAXN=2e5+30,MAX_LOG=30,MOD=1e9+7,INF=1e17;
const double PI = acos(-1);
int mod(int x) { return (x % MOD + MOD) % MOD; }
int n, m, q;

// int a[MAXN];
// int pos[MAXN];
// int dist_to[MAXN];

// int used_val[MAXN];
// void dijkstra(int src){
//     for (int i = 0; i < MAXN; i++){
//         dist_to[i] = INF;
//     }
    
//     priority_queue<pii, vector<pii>, greater<pii> > q;
//     dist_to[src] = 0;
//     q.push(make_pair(0, src));

//     while(!q.empty()){
//         int curr_dist = q.top().first;
//         int curr = q.top().second;
//         q.pop();

//         if(curr_dist != dist_to[curr]){
//             continue;
//         }

//         if(curr == n){
//             break;
//         }

//         if(curr < n && dist_to[curr + 1] > curr_dist + 1){
//             dist_to[curr + 1] = curr_dist + 1;
//             q.push(make_pair(dist_to[curr + 1], curr + 1));
//         }

//         int val = a[curr];
//         if(!used_val[val]){

//             used_val[val] = true;

//             for(int i = val; i <= n; i += val){
//                 int nxt = pos[i];
//                 if(dist_to[nxt] > curr_dist + 1){
//                     dist_to[nxt] = curr_dist + 1;
//                     q.push(make_pair(dist_to[nxt], nxt));
//                 }
//             }
//         }
//     }
// }

vi g[MAXN];
int visited[MAXN];
int deg[MAXN];
int src;
void dfs(int v, vi &order){
    visited[v] = 1;
    for(int u:g[v]){
        if(!visited[u]){
            dfs(u, order);
        }
    }
    if(deg[v] == 1  && v!=src){
    order.PB(v);
    }
}

int MAIN(){
    cin>>n;
    for(int i=1; i <= n-1; i++){
        int u,v;
        cin>>u>>v;
        deg[u]++;
        deg[v]++;
        g[u].PB(v);
        g[v].PB(u);
    }
    for (int i = 1; i <= n; i++){
        if(deg[i] > 1){
            src = i;
            break;
        }
    }
    vi orders;
    dfs(src, orders);

    int leaf_cnt = (int) orders.size();
    int ans = (leaf_cnt + 1) / 2;
    cout<<ans<<endl;
    for(int i = 0; i < ans; i++){
        cout<<orders[i] << ' ' << orders[(i + ans) % leaf_cnt]<<endl;
    }
    
    
    return 0;
}

int32_t main(){
    IOS;
    int t=1;
    //cin>>t;
    while(t--)MAIN();
    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED