static List<int>? TopologicalOrder(IReadOnlyList<int>[] graph)
{
    var indegree = new int[graph.Length];
    foreach (IReadOnlyList<int> edges in graph)
    {
        foreach (int next in edges) indegree[next]++;
    }

    var queue = new Queue<int>();
    for (int vertex = 0; vertex < graph.Length; vertex++)
    {
        if (indegree[vertex] == 0) queue.Enqueue(vertex);
    }

    var order = new List<int>();
    while (queue.TryDequeue(out int current))
    {
        order.Add(current);

        foreach (int next in graph[current])
        {
            indegree[next]--;
            if (indegree[next] == 0) queue.Enqueue(next);
        }
    }

    return order.Count == graph.Length ? order : null;
}