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 don't think your problem is with the function that's summing the array, it's probably that you're summing the array WAY to frequently. If you simply sum the WHOLE array once, and then step through the array until you find the first equilibrium point you should decrease the execution time sufficiently.

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

    long lower = 0;
    long upper = 0;
    foreach (int i in A)
        upper += i;

    for (int i = 0; i < A.Length; i++)
    {
        upper -= A[i];
        if (upper == lower)
        {
            equi = i;
            break;
        }
        else
            lower += A[i];
    }

    return equi;
}