public MaxHeap <int> CreateMaxHeapFromArray(int[] arr)
        {
            MaxHeap <int> heap = new MaxHeap <int>();

            foreach (var item in arr)
            {
                heap.Insert(item);
            }

            return(heap);
        }
        public void Insert_PassNullItem_ThrowsArgumentNullException()
        {
            MaxHeap <string> heap = new MaxHeap <string>();

            Assert.Throws <ArgumentNullException>(() => heap.Insert(null));
        }