Convert Sorted List to Binary Search Tree (Leetcode 109)

Problem Link: https://leetcode.com/problems/convert-sorted-list-to-binary-search-tree/

class Solution:
    def sortedListToBST(self, head: Optional[ListNode]) -> Optional[TreeNode]:
        if(head is None):
            return None
        elif(head.next is None):
            return TreeNode(head.val)
        prev=ListNode(-1)
        prev.next=head
        s=head
        f=head
        while(f is not None and f.next is not None):
            s=s.next
            prev=prev.next
            f=f.next.next
        rootnodeval=s.val
        r=s.next
        s.next=None
        prev.next=None
        l=head
        temp=l
        rootnode=TreeNode(rootnodeval)
        l=self.sortedListToBST(l)
        r=self.sortedListToBST(r)
        rootnode.left=l
        rootnode.right=r
        return rootnode