Building Teams

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

using namespace std;

const int A = 0;
const int B = 1;

bool dfs(int p, int set_team, vector<int> &teams,
         const vector<vector<int>> &edges, set<int> &to_visit) {
  // imp.
  if (teams[p] == !set_team)
    return false;

  // visited
  // if (to_visit.find(p) == to_visit.end())
  //   return teams[p] == set_team;
  to_visit.erase(p);
  teams[p] = set_team;

  // recurse
  for (const auto &f : edges[p]) {
    if (to_visit.find(f) != to_visit.end()) {
      if (!dfs(f, !set_team, teams, edges, to_visit)) {
        return false;
      }
    } else if (teams[f] == set_team) {
      return false;
    }
  }
  return true;
}

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

  vector<int> teams(n);
  vector<vector<int>> edges(n);
  set<int> to_visit;
  for (int i = 0; i < n; ++i) {
    edges.push_back({});
    to_visit.insert(i);
  }
  fill(teams.begin(), teams.end(), -1);
  for (int j = 0; j < m; ++j) {
    int a, b;
    cin >> a >> b;
    edges[a - 1].push_back(b - 1);
    edges[b - 1].push_back(a - 1);
  }

  while (!to_visit.empty()) {
    if (!dfs(*to_visit.begin(), 0, teams, edges, to_visit)) {
      cout << "IMPOSSIBLE" << endl;
      return 0;
    }
  }
  for (int i = 0; i < n; ++i) {
    cout << teams[i] + 1;
  }
  cout << endl;

  return 0;
}

Test 1

Verdict: WRONG ANSWER

Test 2

Verdict: WRONG ANSWER

Test 3

Verdict: WRONG ANSWER

Test 4

Verdict: WRONG ANSWER

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