how does mergesort work with linked list code example

Example: merge sort in linked list

Merge_Sort(head_reference)

STEP 1: If head is NULL or there is only one element in the Linked List 
    then return the Linked List
    
STEP 2: Divide the linked list into two equal halves.  
      Split_Linked_List(head, &first_half, &second_half);
      
STEP 3: Sort the two halves first_half and second_half.
      MergeSort(first_half);
      MergeSort(second_half);
      
STEP 4: Merge the sorted first_half and second_half (using Merge_Sort() recursively) 
   and update the head pointer using head_reference.
     *head_reference = Merge_Sort(first_half, second_half);