Merge Sort on Doubly Linked List

/*
Structure of the node of the list is as
struct Node
{
	int data;
	struct Node *next, *prev;
	Node (int x){
	    data = x;
	    next = prev = NULL;
	}
}; */


// Function to merge two DLLs
Node* split(Node *head){
    Node *fast=head,*slow=head;
    while(fast->next && fast->next->next){
        slow=slow->next;
        fast=fast->next->next;
    }
    Node *tmp=slow->next;
    slow->next=NULL;
    return tmp;
}
Node *merge(Node *first,Node *second){
    if(!first)
        return second;
    if(!second)
        return first;
    if(first->data<second->data){
        first->next=merge(first->next,second);
        first->next->prev=first;
        first->prev=NULL;
        return first;
    }else{
        second->next=merge(first,second->next);
        second->next->prev=second;
        second->prev=NULL;
        return second;
    }
}
struct Node *sortDoubly(struct Node *head)
{
	if(!head || !head->next)
	    return head;
    Node *second=split(head);
    head=sortDoubly(head);
    second=sortDoubly(second);
    
    return merge(head,second);
}