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input/code.cpp: In function 'void bfs(int, std::vector<Node>&, std::vector<std::vector<int> >&)':
input/code.cpp:27:41: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   27 |                 for (int edge = 0; edge < nodes[v].edges.size(); edge++)
      |                                    ~~~~~^~~~~~~~~~~~~~~~~~~~~~~
input/code.cpp: In function 'int main()':
input/code.cpp:57:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<Node>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   57 |         for (int i = 0; i < nodes.size(); i++)
      |                         ~~^~~~~~~~~~~~~~
input/code.cpp:63:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::vector<int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   63 |         for (int i = 0; i < trees.size() - 1; i++)
      |...

#include <iostream>
#include <vector>
#include <queue>

struct Node
{
	public:
		Node(int n) : n(n){}
		
		int n;
		std::vector<int> edges;
		bool discovered = false;
};

void bfs(int i, std::vector<Node>& nodes, std::vector<std::vector<int>>& trees)
{
	std::queue<int> q;
	q.push(i);
	trees.push_back({});
	while (!q.empty())
	{
		int v = q.front();
		q.pop();
		nodes[v].discovered = true;
		trees[trees.size()-1].push_back(v);

		for (int edge = 0; edge < nodes[v].edges.size(); edge++)
		{
			if (nodes[nodes[i].edges[edge]].discovered)
				continue;
			q.push(nodes[i].edges[edge]);
		}
		
		
	}
}

int main()
{
	int n, m;
	std::cin >> n >> m;
	std::vector<Node> nodes;
	for (int i = 0; i < n; i++)
	{
		nodes.push_back(Node(n));
	}
	
	for (int i = 0; i < m; i++)
	{
		int a, b;
		std::cin >> a >> b;
		a --; b--;
		nodes[a].edges.push_back(b);
		nodes[b].edges.push_back(a);
	}
	std::vector<std::vector<int>> trees;
	for (int i = 0; i < nodes.size(); i++)
	{
		if (nodes[i].discovered) continue;
		bfs(i, nodes, trees);
	}
	std::cout << trees.size() - 1 << "\n";
	for (int i = 0; i < trees.size() - 1; i++)
	{
		std::cout << trees[i][0] << " " << trees[i + 1][0] << "\n";
	}

	return 0;
}

Test 1

Verdict: WRONG ANSWER

Test 2

Verdict: RUNTIME ERROR

Test 3

Verdict: WRONG ANSWER

Test 4

Verdict: WRONG ANSWER

Test 5

Verdict: WRONG ANSWER

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: WRONG ANSWER

Test 12

Verdict: WRONG ANSWER