Building Teams

#include <iostream>
#include <vector>
using namespace std;

int main() {
  int n, m;
  cin >> n >> m;
  vector<vector<int>> adj(n + 1);
  for (int i = 0; i < m; i++) {
    int a, b;
    cin >> a >> b;
    adj[a].push_back(b);
    adj[b].push_back(a);
  }

  vector<int> team(n + 1, 0);
  vector<int> stack;
  for (int s = 1; s <= n; s++) {
    if (team[s]) continue;
    team[s] = 1;
    stack.push_back(s);
    while (!stack.empty()) {
      int u = stack.back();
      stack.pop_back();
      for (int v : adj[u]) {
        if (!team[v]) {
          team[v] = 3 - team[u];
          stack.push_back(v);
        } else if (team[v] == team[u]) {
          cout << "IMPOSSIBLE\n";
          return 0;
        }
      }
    }
  }

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