public void ChangeValueTest()
        {
            var heap = new MaxHeap <int, int>();

            heap.Add(3, 5);
            heap.ChangeValue(3, 10);
            Assert.AreEqual(10, heap[3]);
        }
        public void ChangeValueShouldThrow()
        {
            var heap = new MaxHeap <int, int>();

            Assert.Throws <KeyNotFoundException>(() => heap.ChangeValue(3, 10));
        }