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input/code.cpp: In function 'int main()':
input/code.cpp:34:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   34 |     for (int i = 0; i < group.size(); i++)
      |                     ~~^~~~~~~~~~~~~~

// Online C++ compiler to run C++ program online
#include <iostream>
#include <vector>
#include <unordered_set>

int find(int index, std::vector<int>& group)
{
    while (group[index] != index)
        index = group[index];
    
    return index;
}
bool Union(int i1, int i2, std::vector<int>& group)
{
    int i1Root = find(i1, group);
    
    while (group[i2] != i2)
    {
        int prev = group[i2];
        group[i2] = i1Root;
        i2 = prev;
    }
    bool wereInSameGroup = i2 == i1Root;
    group[i2] = i1Root;
    
    return !wereInSameGroup;
}

int main() {
    int n, m;
    std::cin >> n >> m;
    std::vector<int> group = std::vector<int>(n);
    int groupCount = n;
    for (int i = 0; i < group.size(); i++)
        group[i] = i;
    
    for (int i = 0; i < m; i++)
    {
        int a, b;
        std::cin >> a >> b;
        if (Union(a - 1, b - 1, group))
            groupCount--;
        
        
    }
    std::cout << groupCount - 1 << std::endl;
    std::unordered_set<int> sets = {};
    sets.insert(find(0, group));
    for (int i = 1; i < n; i++)
    {
        if (sets.find(find(i, group)) == sets.end())
        {
            std::cout << 1 << " " << i + 1 << std::endl;
            sets.insert(find(i, group));
        }
    }

    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