@Chhotu I think we can implement merge sort inplace using linked list. For merging we can do it in O(nlogn), see this: http://www.geeksforgeeks.org/merge-two-sorted-lists-place/

One change is in the sort procedure where we need to find the middle element, when using arrays we can simply find it in O(1). But in linked list it will take O(n). I think that won't affect the recursive equation. http://www.geeksforgeeks.org/merge-sort-for-linked-list/