private class SkipList<T> : IEnumerable<T>
        {
            private Node<T> head;
            private Node<T> bottomHead;
            public int Count { get { return count; } }
            private int count;
            private Random r;
            Comparer<T> comparer;
            public T Min { get { return bottomHead.Next.Val; } }

            public SkipList()
            {
                comparer = Comparer<T>.Default;
                r = new Random();
                count = 0;
                head = new Node<T>();
                bottomHead = head;
            }

            public void Add(T val)
            {
                Node<Node<T>> savedHead = null;

                // Find insert location
                var node = head;
                while (true)
                {
                    if (node.Next != null && comparer.Compare(node.Next.Val, val) != 1)
                        node = node.Next;
                    else if (node.Down != null)
                    {
                        savedHead = new Node<Node<T>>(node, savedHead, null);
                        node = node.Down;
                    }
                    else
                        break;
                }

                // Add node
                var newNode = new Node<T>(val, node.Next, null);
                node.Next = newNode;
                // Add layers until coinflip is lost
                while (r.Next(0, 2) == 1)
                {
                    var newerNode = new Node<T>(val, null, newNode);
                    if (savedHead != null)
                    {
                        newerNode.Next = savedHead.Val.Next;
                        savedHead.Val.Next = newerNode;
                        savedHead = savedHead.Next;
                    }
                    else
                    {
                        var newHead = new Node<T>(default(T), newerNode, head);
                        head = newHead;
                    }
                    newNode = newerNode;
                }
                ++count;
            }

            public bool Remove(T val)
            {
                Node<Node<T>> savedHead = null;
                bool deletedOne = false;
                // Find node that points to the ndoe we want to delete
                var node = head;
                while (node != null)
                {
                    while (true)
                    {
                        if (node.Next != null && comparer.Compare(node.Next.Val, val) == -1)
                            node = node.Next;
                        else if (node.Next != null && comparer.Compare(node.Next.Val, val) == 0)
                            break;
                        else if (node.Down != null)
                        {
                            savedHead = new Node<Node<T>>(node, savedHead, null);
                            node = node.Down;
                        }
                        else
                            break;
                    }

                    if (node.Next == null || comparer.Compare(node.Next.Val, val) != 0) break;
                    deletedOne = true;
                    node.Next = node.Next.Next;
                    node = node.Down;
                }
                --count;
                return deletedOne;
            }

            public bool Contains(T val)
            {
                throw new NotImplementedException();
            }

            public override string ToString()
            {
                var sb = new StringBuilder();
                var levelNode = head;
                while (levelNode != null)
                {
                    var node = levelNode.Next;
                    while (node != null)
                    {
                        sb.Append(node.Val + "->");
                        node = node.Next;
                    }
                    levelNode = levelNode.Down;
                    sb.Append('\n');
                }
                return sb.ToString();
            }

            public IEnumerator<T> GetEnumerator()
            {
                var node = bottomHead;
                while (node.Next != null)
                {
                    node = node.Next;
                    yield return node.Val;
                }
            }

            IEnumerator IEnumerable.GetEnumerator()
            {
                return this.GetEnumerator();
            }

            private class Node<T>
            {
                public T Val;
                public Node<T> Next;
                public Node<T> Down;
                public Node() { }
                public Node(T val, Node<T> next, Node<T> down)
                {
                    Val = val; Next = next; Down = down;
                }
            }
        }