private static bool CompareIterative(BTN n1, BTN n2)
        {
            // check for nulls
            if (n1 == null || n2 == null)
            {
                return n1 == n2;
            }

            var q1 = new Queue<BTN>();
            var q2 = new Queue<BTN>();

            q1.Enqueue(n1);
            q2.Enqueue(n2);

            while (q1.Count > 0 && q2.Count > 0)
            {
                // get nodes and compare them
                var tempNode1 = q1.Dequeue();
                var tempNode2 = q2.Dequeue();

                if (tempNode1.Val != tempNode2.Val)
                {
                    return false;
                }

                // enqueue Left children of both nodes
                if (tempNode1.Left != null && tempNode2.Left != null)
                {
                    q1.Enqueue(tempNode1.Left);
                    q2.Enqueue(tempNode2.Left);
                }
                else if (tempNode1.Left == null && tempNode2.Left == null)
                {
                    // do nothing
                }
                else if (tempNode1.Left == null || tempNode2.Left == null)
                {
                    return false;
                }

                // enqueue Right children of both nodes
                if (tempNode1.Right != null && tempNode2.Right != null)
                {
                    q1.Enqueue(tempNode1.Right);
                    q2.Enqueue(tempNode2.Right);
                }
                else if (tempNode1.Left == null && tempNode2.Left == null)
                {
                    // do nothing
                }
                else if (tempNode1.Right == null || tempNode2.Right == null)
                {
                    return false;
                }
            }

            return true;
        }

        private static bool CompareRecursive(BTN n1, BTN n2)
        {
            // check for nulls
            if (n1 == null || n2 == null)
            {
                return n1 == n2;
            }

            // check data of the root nodes
            if (n1.Val != n2.Val)
            {
                return false;
            }

            // check left sub trees recursively
            var leftIsEquals = CompareRecursive(n1.Left, n2.Left);

            // check right sub trees recursively
            var rightIsEquals = CompareRecursive(n1.Right, n2.Right);


            // result
            return leftIsEquals && rightIsEquals;
        }