Bus Routes

class Solution {
    class Node{
        int stop;
        int level;
        
        public Node(int stop, int level){
            this.stop = stop;
            this.level = level;
        }
    }
    public int numBusesToDestination(int[][] routes, int source, int target) {
        if(source == target) return 0;
        Queue<Node> q = new LinkedList<>();
        Set<Integer> visited = new HashSet<>();
        Map<Integer, List<Integer>> stopToRoutes = new HashMap<>();
        for(int i = 0; i < routes.length; i++){
            for(int j = 0; j < routes[i].length; j++){
                
                // create adjList between each stop and the routes that stop is in
                List<Integer> listOfRoutes = stopToRoutes.getOrDefault(routes[i][j], new ArrayList());
                stopToRoutes.put(routes[i][j], listOfRoutes);
                listOfRoutes.add(i);
            }
        }
        
        // add the starting stop and level as 0
        q.add(new Node(source, 0));
        
        while(!q.isEmpty()){
            Node curr = q.poll();
            int stop = curr.stop;
            // if this is the destination then return the current level
            if(stop == target){
                return curr.level;
            }
            // add the stop to the visited set so that we don't visit this stop again
            visited.add(stop);
            // get all routes that can be accessed from this stop
            for(Integer route : stopToRoutes.get(stop)){
                // get all possibleStop on this route
                for(int possibleStop : routes[route]){
                    // if this stop hasn't been visited, add it to the q to visit it
                    if(!visited.contains(possibleStop)){
                        q.add(new Node(possibleStop, curr.level + 1));
                    }
                }
                // getting tle without this
                // once a route has been explored, we've seen all stops in this route, so we can set it to empty
                routes[route] = new int[]{};
            }
        }
        
        return -1;
    }
}