public interface IStack
    {
        void Push(StackNode node);
        StackNode Pop();
    }

    public class Stack : IStack
    {
        private LinkedList<StackNode> _elements;

        public Stack()
        {
            _elements = new LinkedList<StackNode>();
        }

        public StackNode Pop()
        {
            if (_elements.Count == 0)
            {
                return null;
            }
            var last = _elements.Last.Value;
            _elements.RemoveLast();
            return last;
        }

        public void Push(StackNode node)
        {
            _elements.AddLast(node);
        }
    }