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

Here is a thought:

private static ArrayList equi(int[] A)
{
    ArrayList answer = new ArrayList();

    //if(A == null) return -1; 
    if ((answer.Count == null))
    {
        answer.Add(-1);
        return answer;
    }

    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) answer.Add(i);
    //return i;
    }
    //return -1;
    return answer;
}