Building Teams

#include <iostream>
#include <vector>

int main()
{
    int n_pupils, n_friendships;
    std::cin >> n_pupils >> n_friendships;
    
    std::vector<std::vector<int>> adjacencies(n_pupils);
    std::vector<bool> teams(n_pupils, false);
    std::vector<bool> init(n_pupils, false);
    std::vector<bool> discovered(n_pupils, false);

    init[0] = true;
    
    for (int i = 0; i < n_friendships; i++)
    {
        int a, b;
        std::cin >> a >> b;
        adjacencies[a].push_back(b);
        adjacencies[b].push_back(a);
    }

    std::vector<int> dfs_stack = {0};
    while (!dfs_stack.empty())
    {
        int v = dfs_stack.back();
        if (discovered[v])
            continue;
        discovered[v] = true;
        dfs_stack.pop_back();
        bool team = teams[v];
        for (int adjacency : adjacencies[v])
        {
            if (init[adjacency] && teams[adjacency] == team)
            {
                std::cout << "IMPOSSIBLE";
                return 0;
            }
            dfs_stack.push_back(adjacency);
            init[adjacency] = true;
            teams[adjacency] = !team;
        }
    }

    for (bool team : teams)
    {
        std::cout << (team ? "1" : "2") << std::endl;
    }
}

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: WRONG ANSWER

Test 7

Verdict: WRONG ANSWER

Test 8

Verdict: WRONG ANSWER

Test 9

Verdict: WRONG ANSWER

Test 10

Verdict: WRONG ANSWER

Test 11

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::bad_alloc'
  what():  std::bad_alloc

Test 12

Verdict: RUNTIME ERROR