Connect cities

input/code.cpp: In function 'int main(int, char**)':
input/code.cpp:11:10: warning: ignoring return value of 'int scanf(const char*, ...)' declared with attribute 'warn_unused_result' [-Wunused-result]
   11 |     scanf("%d %d",&n,&m);
      |     ~~~~~^~~~~~~~~~~~~~~
input/code.cpp:22:14: warning: ignoring return value of 'int scanf(const char*, ...)' declared with attribute 'warn_unused_result' [-Wunused-result]
   22 |         scanf("%d %d", &a, &b);
      |         ~~~~~^~~~~~~~~~~~~~~~~

#include <queue>
#include <stdio.h>
#include <stdlib.h>
#include <unordered_set>
#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>());
    unordered_set<int> unvisited;

    for (int i = 0; i < n; i++) {
        unvisited.insert(i);
    }

    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);
    }

    vector<pair<int, int>> storage(0);
    queue<int> q;
    int last_unvisited = 0;
    do{
        q.push(last_unvisited);
        while (!q.empty()) {
            int current = q.front();
            for (const int &i : adj[current]) {
                if(unvisited.find(i) != unvisited.end()){
                    q.push(i);
                }
            } 
            unvisited.erase(unvisited.find(current));
            q.pop();
        }

        if (unvisited.size() > 0){
            int to = *unvisited.begin();
            unvisited.erase(unvisited.begin());
            adj[0].push_back(to);
            storage.push_back(pair(to, 0));
            last_unvisited = to;
        } 

    } while(unvisited.size() > 0);

    printf("%d\n", (int)storage.size());
    for (pair<int, int>& i : storage) {
        printf("%d %d\n", i.first + 1, i.second +1 );
    }

    return 0;
}

Test 1

Verdict: RUNTIME ERROR

Test 2

Verdict: RUNTIME ERROR

Test 3

Verdict: RUNTIME ERROR

Test 4

Verdict: RUNTIME ERROR

Test 5

Verdict: RUNTIME ERROR

Test 6

Verdict: RUNTIME ERROR

Test 7

Verdict: RUNTIME ERROR

Test 8

Verdict: RUNTIME ERROR

Test 9

Verdict: RUNTIME ERROR

Test 10

Verdict: RUNTIME ERROR

Test 11

Verdict: RUNTIME ERROR

Test 12

Verdict: RUNTIME ERROR