Fragile network

input/code.cpp: In function 'void dfs(long long int, long long int)':
input/code.cpp:58:20: warning: statement has no effect [-Wunused-value]
   58 | #define debug(...) 42
      |                    ^~
input/code.cpp:68:5: note: in expansion of macro 'debug'
   68 |     debug(u, p);
      |     ^~~~~
input/code.cpp:58:20: warning: statement has no effect [-Wunused-value]
   58 | #define debug(...) 42
      |                    ^~
input/code.cpp:77:9: note: in expansion of macro 'debug'
   77 |         debug(adj[u]);
      |         ^~~~~

#include <bits/stdc++.h>
using namespace std;
 
#define int long long
 
#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

string to_string(string s) {
    return '"' + s + '"';
}
 
string to_string(const char* s) {
    return to_string((string) s);
}
 
string to_string(bool b) {
    return (b ? "true" : "false");
}
 
template <typename A, typename B>
string to_string(pair<A, B> p) {
    return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
 
template <typename A>
string to_string(A v) {
    bool first = true;
    string res = "{";
    for (const auto &x : v) {
        if (!first) {
            res += ", ";
        }
        first = false;
        res += to_string(x);
    }
    res += "}";
    return res;
}
 
void debug_out() {
    cerr << endl;
}
 
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
    cerr << " " << to_string(H);
    debug_out(T...);
}
 
#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

const int N = 1e5 + 5;
set<int> adj[N];
int n, nn;
set<int> alive;
bool vis[N];

void dfs(int u, int p) {
    debug(u, p);
    if (sz(adj[u]) == 2 && p) {
        if (adj[p].count(u)) adj[p].erase(u);
        int v = *adj[u].begin();
        if (adj[v].count(u)) adj[v].erase(u);
        nn--;
        alive.erase(u);
        dfs(v, p);
    } else {
        debug(adj[u]);
        if (adj[u].empty()) return;
        vector<int> ele(all(adj[u]));
        for (int v : ele) {
            if (vis[v]) continue;
            vis[v] = 1;
            dfs(v, u);
        }
    }
}

signed main() {
    cin.tie(0)->sync_with_stdio(0);
    cin.exceptions(cin.failbit); // RTE if input wrong datatype

    cin >> n;
    nn = n;
    for (int i = 1; i <= n; i++) alive.insert(i);
    for (int i = 1; i < n; i++) {
        int u, v;
        cin >> u >> v;
        adj[u].insert(v);
        adj[v].insert(u);
    }
    
    vis[1] = 1;
    dfs(1, 0);

    if (sz(adj[1]) == 2) {
        int u = *adj[1].begin();
        int v = *next(adj[1].begin());
        if (adj[u].count(1)) adj[u].erase(1);
        if (adj[v].count(1)) adj[v].erase(1);
        adj[u].insert(v);
        adj[v].insert(u);
        nn--;
        alive.erase(1);
    }

    if (nn == 2) {
        cout << 1 << '\n';
        int u = *alive.begin();
        int v = *next(alive.begin());
        cout << u << ' ' << v << '\n';
    } else {
        int root = -1;
        for (int u : alive) {
            if (sz(adj[u]) > 1) {
                root = u;
                break;
            }
        }

        vector<pii> ans;
        vector<int> c(adj[root].begin(), adj[root].end());
        for (int i = 0; i + 1 < sz(c); i += 2) {
            ans.push_back({c[i], c[i + 1]});
        }
        if (sz(c) & 1) {
            ans.push_back({c[sz(c) - 2], c[sz(c) - 1]});
        }

        cout << sz(ans) << '\n';
        for (auto [u, v] : ans) cout << u << ' ' << v << '\n';
    }
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: WRONG ANSWER

Test 3

Verdict: RUNTIME ERROR

Test 4

Verdict: WRONG ANSWER

Test 5

Verdict: WRONG ANSWER

Test 6

Verdict: ACCEPTED

Test 7

Verdict: WRONG ANSWER

Test 8

Verdict: WRONG ANSWER

Test 9

Verdict: WRONG ANSWER

Test 10

Verdict: WRONG ANSWER

Test 11

Verdict: WRONG ANSWER

Test 12

Verdict: WRONG ANSWER

Test 13

Verdict: WRONG ANSWER

Test 14

Verdict: WRONG ANSWER

Test 15

Verdict: RUNTIME ERROR

Test 16

Verdict: WRONG ANSWER

Test 17

Verdict: WRONG ANSWER

Test 18

Verdict: WRONG ANSWER

Test 19

Verdict: WRONG ANSWER

Test 20

Verdict: WRONG ANSWER

Test 21

Verdict: WRONG ANSWER