Implement an algorithm to find the Kth to last element of a singly linked string
To get the sub-list of the last n elements without computing the length first (see comments):


Go ahead with an end pointer until you have skipped n elements. 
Now go ahead in parallel with a start pointer and the end pointer until the end pointer reaches the end of the list (the start and end pointers will stay n elements apart as you go).
Now start points to the desired sub-list.


In an interview, before you implement anything, it's often a good idea to first describe your plan. Counting first works and is no different in terms of the O notation, but I doubt it's what the interviewer wanted to see. Describing your plan can avoid spending time on something the interviewer might not really be interested in.