Building Teams

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;


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

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

    // Read friendships
    for (int i = 0; i < m; ++i) {
        int f1, f2;
        cin >> f1 >> f2;
        friendships[f1].push_back(f2);
        friendships[f2].push_back(f1);
    }

    vector<int> team(n+1, -1);

    for (int i = 1; i <= n; ++i) {
        if (team[i] == -1)
            team[i] = 1;
        if (team[i] == 1) {
            int s = friendships[i].size();
            for (int j = 0; j < s; ++j) {
                int fr = friendships[i][j];
                if (team[fr] == 1) {
                    cout << "IMPOSSIBLE" << endl;
                    return 0;
                }
                else if (team[fr] == -1) team[fr] = 2;
            }
        }
        else {
            int s = friendships[i].size();
            for (int j = 0; j < s; ++j) {
                int fr = friendships[i][j];
                if (team[fr] == 2) {
                    cout << "IMPOSSIBLE" << endl;
                    return 0;
                }
                else if (team[fr] == -1) team[fr] = 1;
            }
        }

    }

    for (int i = 1; i <= n; ++i)
        cout << team[i] << " ";
    cout << endl;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: WRONG ANSWER

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: WRONG ANSWER

Test 7

Verdict: WRONG ANSWER

Test 8

Verdict: WRONG ANSWER

Test 9

Verdict: WRONG ANSWER

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED