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

This got me 100% in Javascript:

function solution(A) {
    if (!(A) || !(Array.isArray(A)) || A.length < 1) {
        return -1;
    }

    if (A.length === 1) {
        return 0;
    }

    var sum = A.reduce(function (a, b) { return a + b; }),
        lower = 0,
        i,
        val;

    for (i = 0; i < A.length; i++) {
        val = A[i];
        if (((sum - lower) - val) === (lower)) {
            return i;
        }
        lower += val;
    }

    return -1;
}