Optional$>$ findRefBefore$($T elem$)$

If the list contains elem, then this function should return a reference to that list node.

If the list does not contain elem, then this function should return a reference to the node that

would immediately come before a node with value elem if it were to be inserted into the list.

If elem is smaller than every element in the list $($ie the elem would be inserted at the head of the

list$)$ then this function should return Optional.empty$().$

For a list of length n$,$ this function should run in O$($n$).$