Building Teams

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

using namespace std;

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

    vector<vector<bool> > friendships(n_pupils, vector<bool>(n_pupils, false));
    vector<bool> visited(n_pupils, false);
    vector<int> team(n_pupils, -1);




    for (int i = 0; i < n_friendships; ++i) {
        int a, b;
        cin >> a >> b;
        friendships[a - 1][b - 1] = true;
        friendships[b - 1][a - 1] = true;
    }

    bool impossible = false;
    for (int i = 0; i < n_pupils; ++i) {

        for (int j = 0; j < n_pupils; ++j) {
            if (friendships[i][j]) {

                if (team[i] != -1 and team[i] == team[j]) impossible = true;
                if (team[i] == 1) team[j] = 2;
                else team[j] = 1;

            }
        
        }

    }

    if (not impossible)
        for (int i = 0; i < n_pupils; ++i) cout << team[i] << " ";
    else
        cout << "IMPOSSIBLE";
    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: RUNTIME ERROR

Test 7

Verdict: RUNTIME ERROR

Test 8

Verdict: RUNTIME ERROR

Test 9

Verdict: RUNTIME ERROR

Test 10

Verdict: RUNTIME ERROR

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED