fix bug on delete operation in doubly linked-list

[amirsoroush]

In "Chapter04/delete_operation_doubly_linkedlist.py" in case where the item to be deleted is found at starting of the list, there is currently a bug:

    def delete(self, data):
        # Delete a node from the list. 
        current = self.head 
        node_deleted = False 
        if current is None:       #List is empty
            print("List is empty")
        elif current.data == data:   #Item to be deleted is found at starting of list
            self.head.prev = None 
            node_deleted = True 
            self.head = current.next

The self.head.prev = None line has no effect because it's already None. Therefore when self.head = current.next line is executed we'll have a head which has a prev!

To fix it all we have to do is move the self.head = current.next line to the top so that after the new head is assigned, we set its prev to None.

[Yashwanth-K]

yes you are right..