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

private static int equi ( int[] A ) {
    if (A == null || A.length == 0)
     return -1;
 long tot = 0;
 int len = A.length;
 for(int i=0;i

}

I considered the array as a bilance so if the equilibrium index exist then half of the weight is on the left. So I only compare the partTot (partial total) x 2 with the total weight of the array. the Alg takes O(n) + O(n)