Fragile network

// #ifdef ONLINE_JUDGE
// #pragma GCC optimize("O3,unroll-loops")
// #pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
// #endif
#include "bits/stdc++.h"
#define fast ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define fill(arr,val) memset(arr,val,sizeof(arr))
#define FOR(i, a, b) for(__typeof(b) i = a, _b = b; i <= _b; ++i)
#define FORD(i, a, b) for(__typeof(a) i = a, _b = b; i >= _b; --i)
#define ALL(a) (a).begin(), (a).end()
#define YES cout << "YES\n"
#define NO cout << "NO\n"
#define ll long long
#define fi first
#define se second
#define pb push_back
#define pf push_front
#define ii pair<int,int>
#define iii pair<int,pair<int,int>>
#define dq deque<int>
#define nend '\n'
using namespace std;

const ll inf = 1e18;
const int dx[] = {1,0,-1,0};
const int dy[] = {0,1,0,-1};
const int MOD = 1e9 + 7;
const ll hashi = 2e9 + 11;

void init_code() {
    fast;
    #ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
    #endif
}
const int N = 1e5 + 3;
int n, root = 1, par[N];
vector<int> g[N], leaves;

// answer = ceil(L/2), L = number of leaves
// list leaves in DFS order from a non-leaf root, join leaf i with leaf i+L/2
// -> removing any tree edge cuts off a contiguous block of leaves, some new edge crosses it
void solve() {
    cin >> n;
    FOR(i,1,n-1) {
        int a,b; cin >> a >> b;
        g[a].pb(b); g[b].pb(a);
    }
    while (g[root].size()==1) root++;

    
    vector<int> st = {root};
    par[root] = -1;
    while (!st.empty()) {
        int u = st.back(); st.pop_back();
        if (g[u].size()==1) leaves.pb(u);
        for (int v : g[u]) if (v != par[u]) { par[v] = u; st.pb(v); }
    }

    int L = leaves.size(), k = (L+1)/2;
    cout << k << nend;
    FOR(i,0,k-1) cout << leaves[i] << ' ' << leaves[i+L/2] << nend;
}

int main() {
    init_code();

    int t = 1;
    // cin >> t;
    while (t--) {
        solve();
    }

    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