class Solution {
    public int[] findOrder(int numCourses, int[][] prerequisites) {
        HashSet<Integer> nodes = new HashSet<>();
        HashMap<Integer, HashSet<Integer>> deps = new HashMap<>();

        for (int i = 0; i < numCourses; i++) {
            nodes.add(i);
            deps.put(i, new HashSet<Integer>());
        }

        for (int[] req: prerequisites) {
            deps.get(req[0]).add(req[1]);
        }

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

        while (nodes.size() > 0){
            HashSet<Integer> out = new HashSet<>();

            for (int node: nodes){
                if (deps.get(node).size() == 0) out.add(node);
            }

            if (out.size() == 0) return new int[0];

            nodes.removeAll(out);
            for (int node: nodes) deps.get(node).removeAll(out);

            res.addAll(out);
        }

        int[] result = new int[numCourses];
        for (int i = 0; i < numCourses; i++) result[i] = res.get(i);
        return result;
    }
}