LeetCode 108. 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.


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:

     / \
   -3   9
   /   /
 -10  5


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).

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