## Description

https://leetcode.com/problems/convert-sorted-array-to-binary-search-tree/

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of *every* node never differ by more than 1.

**Example:**

Given the sorted array: [-10,-3,0,5,9], One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST: 0 / \ -3 9 / / -10 5

## Explanation

find middle as root node and construct left and right trees.

## Python Solution

```
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution:
def sortedArrayToBST(self, nums: List[int]) -> TreeNode:
return self.sort_bst_helper(nums)
def sort_bst_helper(self, nums):
if nums == None or len(nums) == 0:
return None
if len(nums) == 1:
return TreeNode(nums[0])
mid = len(nums) // 2
root = TreeNode(nums[mid])
root.left = self.sort_bst_helper(nums[0: mid])
root.right = self.sort_bst_helper(nums[mid + 1 : len(nums)])
return root
```

- Time complexity: O(N).
- Space complexity: O(N).

It is a best solution found that very popular and helpful:

https://www.youtube.com/watch?v=guyEygPRaiE&ab_channel=EricProgramming