Connect cities

#include <bits/stdc++.h>
using namespace std;
#define int long long
 
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 = 2e5 + 5;
 
int n, m;
int par[N], sz[N];
 
int root(int u) {
    return par[u] == u ? u : par[u] = root(par[u]);
}
 
void unite(int u, int v) {
    u = root(u), v = root(v);
    if (u == v) return;
    if (sz[u] < sz[v]) swap(u, v);
    sz[u] += sz[v];
    par[v] = u;
}
 
signed main() {
    cin.tie(0)->sync_with_stdio(0);
 
    cin >> n >> m;
    for (int i = 1; i <= n; i++) {
        par[i] = i;
        sz[i] = 1;
    }
 
    for (int i = 1; i <= m; i++) {
        int u, v;
        cin >> u >> v;
        unite(u, v);
    }
 
    set<int> st;
    for (int i = 1; i <= n; i++) {
        st.insert(root(i));
    }
 
    vector<int> v(st.begin(), st.end());
    cout << (int)v.size() - 1 << '\n';
    for (int i = 0; i + 1 < (int)v.size(); i++) {
        cout << v[i] << ' ' << v[i + 1] << '\n';
    }
}

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