Connect cities

input/code.cpp: In function 'int main(int, char**)':
input/code.cpp:10:10: warning: ignoring return value of 'int scanf(const char*, ...)' declared with attribute 'warn_unused_result' [-Wunused-result]
   10 |     scanf("%d %d",&n,&m);
      |     ~~~~~^~~~~~~~~~~~~~~
input/code.cpp:17:14: warning: ignoring return value of 'int scanf(const char*, ...)' declared with attribute 'warn_unused_result' [-Wunused-result]
   17 |         scanf("%d %d", &a, &b);
      |         ~~~~~^~~~~~~~~~~~~~~~~

#include <queue>
#include <stdio.h>
#include <stdlib.h>
#include <utility>
#include <vector>
using namespace std;

int main(int argc, char *argv[]) {
    int n,m;
    scanf("%d %d",&n,&m);

    vector<vector<int>> adj(n,vector<int>());
    int *visited = (int *)calloc(n,sizeof(int));

    for (int i = 0; i < m; i++) {
        int a,b;
        scanf("%d %d", &a, &b);
        adj[a-1].push_back(b-1);
        adj[b-1].push_back(a-1);
    }


    int unvisited = -1, lastvisited = 0, exists_unvisited = 0;

    vector<pair<int, int>> storage(0);

    do{
        if (exists_unvisited){
            adj[lastvisited].push_back(unvisited);
            storage.push_back(pair(unvisited, lastvisited));
        } 
        exists_unvisited = 0;
        queue<int> q;

        q.push(lastvisited);

        while (!q.empty()) {
            int current = q.front();
            for (const int &i : adj[current]) {
                if(!visited[i]){
                    q.push(i);
                }
            } 
            visited[current] = 1;
            lastvisited = current;
            q.pop();
        }

        for (int i = 0; i < n; i++) {
            if (visited[i] == 0){ 
                unvisited = i;
                exists_unvisited = 1;
                break;
            }
        }
    } while(exists_unvisited);
    printf("%d\n", (int)storage.size());
    for (pair<int, int>& i : storage) {
        printf("%d %d\n", i.first + 1, i.second +1 );
    }

    free(visited);

    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: TIME LIMIT EXCEEDED

Test 12

Verdict: ACCEPTED