Building Teams

#include<iostream>
#include<queue>
#include <unordered_map>
#include<tuple>

typedef long long ll;
using namespace std;

int main()
{
    ll students, friendships;

    cin >> students >> friendships;

    vector<ll> adj[students + 1];

    ll a, b;
    while (cin >> a >> b)
    {
        adj[a].push_back(b);
        adj[b].push_back(a);
    }

    vector<ll> teams(students + 1, 0);

    ll cluster = 1;

    for (int c = 1; c <= students; ++c)
    {
        if (teams[c] != 0)
        {
            continue;
        }
        else
        {   
            queue<tuple<ll, ll>> q;
            q.push(make_tuple(c, cluster));
            vector<ll> children;

            while (!q.empty())
            {   
                tuple<ll, ll> node = q.front();
                q.pop();
                ll student = get<0>(node);
                ll cluster = get<1>(node);
                teams[student] = cluster;

                for (const auto b : adj[student])
                {   
                    if (teams[b] == 0)
                    {
                        q.push(make_tuple(b, -1*cluster));
                    }
                    else if (teams[b] == cluster)
                    {
                        cout << "IMPOSSIBLE" << '\n';
                        return 0;
                    }
                }
            }
        }
    }

    for (int i = 1; i <= students; ++i)
    {   
        if (teams[i] == -1)
        {
            cout << 1;
        }
        else
        {
            cout << 2;
        }
        cout << endl;
    }


    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