BFS.cs

public HashSet<T> BFS<T>(Graph<T> graph, T start) {
    var visited = new HashSet<T>();

    if (!graph.AdjacencyList.ContainsKey(start))
        return visited;

    var queue = new Queue<T>();
    queue.Enqueue(start);

    while (queue.Count > 0) {
        var vertex = queue.Dequeue();

        if (visited.Contains(vertex))
            continue;

        visited.Add(vertex);

        foreach(var neighbor in graph.AdjacencyList[vertex])
            if (!visited.Contains(neighbor))
                queue.Enqueue(neighbor);
    }

    return visited;
}