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;
    }

    vector<pair<int, int> > teams;

    bool all_visited = false;
    bool impossible = false;
    while (not all_visited and not impossible) {
        all_visited = true;
        int k = 0;
        all_visited = visited[k];
        while (all_visited and k < n_pupils) {
            all_visited = visited[k];
            ++k;
        }

        queue<int> q;
        visited[k] = true;
        team[k] = 1;
        q.push(k);

        while (!q.empty() and not all_visited and not impossible) {
            int curr = q.front();
            q.pop();

            for (int i = 0; i < n_pupils; ++i) {
                if (friendships[curr][i] and team[curr] != -1 and team[curr] == team[i]) impossible = true;

                if (friendships[curr][i] and not visited[i] and not impossible) {
                    q.push(i);
                    visited[i] = true;

                    if (team[curr] == 1)
                        team[i] = 2;
                    else
                        team[i] = 1;
                }
            }
        }
    }

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

Test 1

Verdict: RUNTIME ERROR

free(): invalid pointer

Test 2

Verdict: RUNTIME ERROR

free(): invalid pointer

Test 3

Verdict: RUNTIME ERROR

free(): invalid pointer

Test 4

Verdict: RUNTIME ERROR

free(): invalid pointer

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