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); }