public void UpdateTree_UpdateTreeAndRequestSum_GetSum()
{
int[] array = { 2, 1, 1, 3, 2, 3, 4, 5, 6, 7, 8, 9 };
var tree = new BinaryIndexedTree(array);
var expectedSum = 18;
array[3] += 6;
tree.UpdateTree(3, 6);
var resultedSum = tree.GetSum(5);
resultedSum.Should().Be(expectedSum);
}