CareerCup

Walk second list and insert all elements in a hash.
Walk first list and if an element is already in hash it is the common element.

1) Find lengths of two linked list O(n1) + O(n2)
2) Find the long list and skip the (long list length - short list length) nodes
3) Now walk the two linked lists together (in parallel one step/move-next at a time), and compare the values of two nodes (just compare node address if the lists are already joined). Return the node value/address where the euality check succeeds. O(min(n1, n2))

List1.Intersect(List2).First();

Of course that's not an acceptable answer.
So sort the lists
Insert elements of one list into a HashTable.
Walk the second list and check each element to see if it's in the HashTable.
Return the first or null.