If we have two sorted linked list and we want to merge both in a new list without creating new node space then we can use recursive approach for this . This approach takes O(max(m,n)) time complexity for merging.
Like we have a linked list named : list 1 1 -> 2 -> 3 -> 4
and another list named : list2 5 -> 6 -> 7
then resulting linked list will be 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7