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input/code.cpp: In function 'int main()':
input/code.cpp:40:24: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   40 |         for (int i=0; i<rc.size()-1; i++) {
      |                       ~^~~~~~~~~~~~

#include <bits/stdc++.h>

using namespace std;
const int N = 100005;
vector<int> adj[N];
bool visited[N];

int dfs(int s) {
	if (visited[s]) return 0;
	visited[s] = true;
	int vis = 0;
	for (int nb : adj[s]) {
		vis += dfs(nb);
	}
	return vis + 1;
}

int main() {
	int n, m, a, b;
	cin >> n >> m;
	for (int i=0; i<m; i++) {
		cin >> a >> b;
		adj[a].push_back(b);
		adj[b].push_back(a);
	}
	int dis = 0;
	int vis = 0;
	vector<int> rc;
	for (int i=1; i<=n; i++) {
		int v = dfs(i);
		if (v) {
			dis += 1;
			vis += v;
			rc.push_back(i);
		}
		if (vis == n) break;
	}
	cout << dis-1 << "\n";

	for (int i=0; i<rc.size()-1; i++) {
		cout << rc[i] << " " << rc[i+1] << "\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