# LeetCode 108. Convert Sorted Array to Binary Search Tree

## 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)

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).