ArrayList<int[]> bfs(int start, int end) {
  ArrayList<Integer> q = new ArrayList<Integer>();
  ArrayList<int[]> res = new ArrayList<int[]>();
  q.add(start);
  int[] edge;
  int q_size = 1;
  while (q_size > 0) {
    int next = q.get(0);
    q.remove(0);
    q_size--;
    vis[next] = true;
    g.add_edge(0, 0);
    for (int neighbor : g.adj.get(next)) {
      if (vis[neighbor]) continue;
      q.add(neighbor);
      edge = new int[]{next, neighbor};
      res.add(edge);
      q_size++;
      if (neighbor == end) return res;
    }
  }
  return res;
}