Connect cities

input/code.cpp: In function 'int get_new_city(bool*, size_t)':
input/code.cpp:8:14: warning: comparison of integer expressions of different signedness: 'int' and 'size_t' {aka 'long unsigned int'} [-Wsign-compare]
    8 |     for (; i < size; i++) {
      |            ~~^~~~~~

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;

int get_new_city(bool *visited, size_t size) {
    static int i = 1;
    for (; i < size; i++) {
        if (!visited[i])
            return i;
    }
    return -1;
}

void dfs(int c, bool *v, vector<int> *adj) {
    if (v[c])
        return;
    v[c] = true;

    for (auto nc : adj[c]) {
        dfs(nc, v, adj);
    }
}

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

    int n, m;
    cin >> n >> m;

    bool visited[n+1];
    memset(visited, 0, (n+1) * sizeof(int));

    vector<int> adj[n+1];
    int f, s;
    for (int i = 0; i < m; i++) {
        cin >> f >> s;
        adj[f].push_back(s);
        adj[s].push_back(f);
    }

    vector<pair<int, int>> new_roads;
    int p_city = 0, c_city;
    while ((c_city = get_new_city(visited, n+1)) != -1) {
        if (p_city != 0) {
            new_roads.push_back(make_pair(p_city, c_city));
        }
        p_city = c_city;
        dfs(c_city, visited, adj);
    }

    cout << new_roads.size() << "\n";
    for (auto p : new_roads) {
        cout << p.first << " " << p.second << "\n";
    }

    return 0;
}

Test 1

Verdict: WRONG ANSWER

Test 2

Verdict: WRONG ANSWER

Test 3

Verdict: ACCEPTED

Test 4

Verdict: WRONG ANSWER

Test 5

Verdict: ACCEPTED

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: WRONG ANSWER