faster implementation of sum ( for Codility test )

Question

How can the following simple implementation of sum be faster?

private long sum( int [] a, int begin, int end ) {
    if( a == null   ) {
        ret         


        

Answer 1

100% correctness and performance of this code is tested

Private Function equi(ByVal A() As Integer) As Integer
        Dim index As Integer = -1
        If A.Length > 0 And Not IsDBNull(A) Then
            Dim sumLeft As Long = 0
            Dim sumRight As Long = ArraySum(A)
            For i As Integer = 0 To A.Length - 1
                Dim val As Integer = A(i)

                sumRight -= val
                If sumLeft = sumRight Then
                    index = i
                End If
                sumLeft += val
            Next
        End If

        Return index
    End Function