815. Bus Routes

class Solution {
    public int numBusesToDestination(int[][] routes, int S, int T) {
        Map<Integer, Set<Integer>> buses = new HashMap<>(), route = new HashMap<>();
        int b = 0;
        for(int[] r : routes)
        {
            buses.put(b, new HashSet<>());
            for(int stops : r)
            {
                buses.get(b).add(stops);
                if(!route.containsKey(stops)) route.put(stops, new HashSet<>());
                route.get(stops).add(b);
            }
            b++;
        }
        Queue<Integer> q = new LinkedList<>();
        Set<Integer> set = new HashSet<>();
        // bfs visits, q contains the stops
        q.offer(S);
        int re = 0;
        while(!q.isEmpty())
        {
            int size = q.size();
            for(int i = 0; i < size; i++)
            {
                int stop = q.poll();
                if(stop == T) return re;
                for(int bus : route.get(stop))
                {
                    if(set.contains(bus)) continue;
                    set.add(bus);
                    for(int next : buses.get(bus))
                        q.offer(next);
                }
            }
            re++;
        }
        return -1;
    }
}