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class Solution {

    List<List<Integer>> adjList = new ArrayList();

    // Iterate over each pair of routes and add an edge between them if there's a common stop.
    void createGraph(int[][] routes) {
        for (int i = 0; i < routes.length; i++) {
            for (int j = i + 1; j < routes.length; j++) {
                if (haveCommonNode(routes[i], routes[j])) {
                    adjList.get(i).add(j);
                    adjList.get(j).add(i);
                }
            }
        }
    }

    // Returns true if the provided routes have a common stop, false otherwise.
    boolean haveCommonNode(int[] route1, int[] route2) {
        int i = 0, j = 0;
        while (i < route1.length && j < route2.length) {
            if (route1[i] == route2[j]) {
                return true;
            }

            if (route1[i] < route2[j]) {
                i++;
            } else {
                j++;
            }
        }
        return false;
    }

    // Add all the routes in the queue that have the source as one of the stops.
    void addStartingNodes(Queue<Integer> q, int[][] routes, int source) {
        for (int i = 0; i < routes.length; i++) {
            if (isStopExist(routes[i], source)) {
                q.add(i);
            }
        }
    }

    // Returns true if the given stop is present in the route, false otherwise.
    boolean isStopExist(int[] route, int stop) {
        for (int j = 0; j < route.length; j++) {
            if (route[j] == stop) {
                return true;
            }
        }
        return false;
    }

    public int numBusesToDestination(int[][] routes, int source, int target) {
        if (source == target) {
            return 0;
        }

        for (int i = 0; i < routes.length; ++i) {
            Arrays.sort(routes[i]);
            adjList.add(new ArrayList());
        }

        createGraph(routes);

        Queue<Integer> q = new LinkedList<>();
        addStartingNodes(q, routes, source);

        Set<Integer> vis = new HashSet<Integer>(routes.length);
        int busCount = 1;
        while (!q.isEmpty()) {
            int sz = q.size();

            while (sz-- > 0) {
                int node = q.remove();

                // Return busCount, if the stop target is present in the current route.
                if (isStopExist(routes[node], target)) {
                    return busCount;
                }

                // Add the adjacent routes.
                for (int adj : adjList.get(node)) {
                    if (!vis.contains(adj)) {
                        vis.add(adj);
                        q.add(adj);
                    }
                }
            }

            busCount++;
        }

        return -1;
    }
}