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

In C++, the following:

int* a1 = a + begin;
for( int i = end - begin - 1; i >= 0 ; i-- )
{
    r+= a1[i];
}

might be faster. The advantage is that we compare against zero in the loop.

Of course, with a really good optimizer there should be no difference at all.

Another possibility would be

int* a2 = a + end - 1;
for( int i = -(end - begin - 1); i <= 0 ; i++ )
{
    r+= a2[i];
}

here we traversing the items in the same order, just not comparing to end.