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

100% O(n) solution in C

int equi ( int A[], int n ) {

    long long sumLeft = 0;
    long long sumRight = 0;
    int i;

    if (n <= 0) return -1;

    for (i = 1; i < n; i++)
        sumRight += A[i];
    i = 0;

    do {
        if (sumLeft == sumRight)
            return i;

        sumLeft += A[i];

        if ((i+1) < n)
            sumRight -= A[i+1];
        i++;
    } while (i < n);

    return -1;
}

Probably not perfect but it passes their tests anyway :)

Can't say I'm a big fan of Codility though - it is an interesting idea, but I found the requirements a little too vague. I think I'd be more impressed if they gave you requirements + a suite of unit tests that test those requirements and then asked you to write code. That's how most TDD happens anyway. I don't think doing it blind really gains anything other than allowing them to throw in some corner cases.