Connect cities

#include "bits/stdc++.h"

using namespace std;

typedef long long ll;
typedef pair<int, int> pii;

#define F first
#define S second
#define PB push_back
#define MP make_pair

#define REP(i,a,b) for (int i = a; i < b; i++)

void visit(vector<vector<int>> &edges, vector<bool> &visited, int n) {
    if(visited[n] == true) return;
    visited[n] = true;

    for(auto e: edges[n]) {
        visit(edges, visited, e);
    }
}

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);

    int n, m;
    cin >> n >> m;
    vector<int> nodes(n);
    vector<vector<int>> edges(n);

    REP(i, 0, n) {
        nodes[i] = i+1;
    }

    REP(i, 0, m) {
        int a,b;
        cin >> a >> b;
        edges[a-1].PB(b-1);
        edges[b-1].PB(a-1);
    }

    vector<int> need;
    vector<bool> visited(n);

    int next = 0;

    while(next < n) {

        visit(edges, visited, next);
        need.PB(nodes[next]);

        while(next < n && visited[next] == true) {
            ++next;
        }



    }

    int nr = need.size() - 1;

    cout << nr << "\n";
    REP(i, 1, (int)need.size()) {
        cout << need[i-1] << " " << need[i] << "\n";
    }
    
    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