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

If this is based on the actual sample problem, your issue isn't the sum. Your issue is how you calculate the equilibrium index. A naive implementation is O(n^2). An optimal solution is much much better.