public static class TreeHelper
    {
        public static IEnumerable<T> GetDepthFirstTree<T>(T root, Func<T, IEnumerable<T>> childrenSelector)
        {
            var stack = new Stack<T>();
            stack.Push(root);
            while (stack.Count > 0)
            {
                var item = stack.Pop();
                yield return item;
                
                var children = childrenSelector(item);
                if (children != null)
                    children.Reverse().Push(stack);
            }
        }

        public static IEnumerable<T> GetBreadthFirstTree<T>(T root, Func<T, IEnumerable<T>> childrenSelector)
        {
            var queue = new Queue<T>();
            queue.Enqueue(root);
            while (queue.Count > 0)
            {
                var item = queue.Dequeue();
                yield return item;

                var children = childrenSelector(item);
                if (children != null)
                    children.Enqueue(queue);
            }
        }        
    }

    public static class EnumerableHelper
    {        
        public static void Enqueue<T>(this IEnumerable<T> src, Queue<T> dest)
        {
            foreach (var item in src)
                dest.Enqueue(item);
        }

        public static void Push<T>(this IEnumerable<T> src, Stack<T> dest)
        {
            foreach (var item in src)
                dest.Push(item);
        }
    }