Building Teams

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

using namespace std;

int main() {
    int n_pupils, n_friendships;
    cin >> n_pupils >> n_friendships;

    vector<vector<int> > friendships(n_pupils);
    for (int i = 0; i < n_friendships; i++) {
        int a, b;
        cin >> a >> b;
        a--; b--;
        friendships[a].push_back(b);
        friendships[b].push_back(a);
    }

    vector<int> team(n_pupils, -1);

    for (int i = 0; i < n_pupils; i++) {
        if (team[i] == -1) {
            queue<int> q;
            q.push(i);
            team[i] = 1;

            while (!q.empty()) {
                int curr = q.front();
                q.pop();

                for (int neighbor : friendships[curr]) {
                    if (team[neighbor] == -1) {
                        team[neighbor] = 3 - team[curr];
                        q.push(neighbor);
                    } else if (team[neighbor] == team[curr]) {
                        cout << "IMPOSSIBLE" << endl;
                        return 0;
                    }
                }
            }
        }
    }

    for (int i = 0; i < n_pupils; i++) {
        cout << team[i] << " ";
    }
    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