Leetcode: Convert Sorted List to Binary Search Tree @Python

# Definition for a  binary tree node
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
#
# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    # @param head, a list node
    # @return a tree node
    def AtoBST(self,A):
        n=len(A)
        if n==0:
            return None
        else:
            ml=n//2
            A[ml].right=self.AtoBST(A[ml+1:])
            A[ml].left=self.AtoBST(A[:ml])
            return A[ml]
    def sortedListToBST(self, head):
        A=[]
        while head:
            A.append(TreeNode(head.val))
            head=head.next
        return self.AtoBST(A)