Building Teams

#include <iostream>
#include <vector>
#include <algorithm>
#include <queue>
#include <unordered_set>
#include <unordered_map>

using namespace std;

bool dfs(int pupil, vector<int> &assignments, const vector<vector<int>> &friends, int team);

int main()
{
    int n, m;
    cin >> n >> m;

    vector<vector<int>> friends(n + 1);

    for (int i = 0; i < m; i++)
    {
        int a, b;
        cin >> a >> b;
        friends[a].push_back(b);
        friends[b].push_back(a);
    }

    vector<int> assignments(n + 1, 0);

    for (int pupil = 1; pupil <= n; pupil++)
    {
        if (assignments[pupil] == 0)
        {
            bool result = dfs(pupil, assignments, friends, 1);
            if (result == false)
            {
                cout << "IMPOSSIBLE\n";
                return 0;
            }
        }
    }

    for (int i = 1; i <= n; i++)
    {
        cout << assignments[i] << " ";
    }
    cout << "\n";
}

bool dfs(int pupil, vector<int> &assignments, const vector<vector<int>> &friends, int team)
{
    int other_team = team == 1 ? 2 : 1;
    if (assignments[pupil] == other_team)
    {
        return false;
    }
    assignments[pupil] = team;
    for (int f : friends[pupil])
    {
        int assignment_friend = assignments[f];
        if (assignment_friend == team)
        {
            return false;
        }
        else if (assignment_friend == 0)
        {
            dfs(f, assignments, friends, other_team);
        }
    }
    return true;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: WRONG ANSWER

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: WRONG ANSWER