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

I have scored 100% for this one:

int equi (int[] A)
{
    if (A == null) return -1;

    long sum0 = 0, sum1 = 0;

    for (int i = 0; i < A.Length; i++) sum0 += A[i];

    for (int i = 0; i < A.Length; i++)
    {
        sum0 -= A[i];
        if (i > 0)
        {
            sum1 += A[i - 1];
        }          
        if (sum1 == sum0) return i;      
    }        
    return -1;
}