convert_sorted_array_to_binary_search_tree.py

# Definition for a binary tree node.
class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right


from typing import List, Optional


class Solution:
    def sortedArrayToBST(self, nums: List[int]) -> Optional[TreeNode]:
        def helper(l, r):
            if l > r:
                return None
            m = (l + r) // 2
            root = TreeNode(nums[m])
            root.left = helper(l, m - 1)
            root.right = helper(m + 1, r)
            return root

        return helper(0, len(nums) - 1)