LeetCode Top Interview 150

108. Convert Sorted Array to Binary Search Tree

Easy

Given an integer array nums where the elements are sorted in ascending order, convert it to a height-balanced binary search tree.

A height-balanced binary tree is a binary tree in which the depth of the two subtrees of every node never differs by more than one.

Example 1:

Input: nums = [-10,-3,0,5,9]

Output: [0,-3,9,-10,null,5]

Explanation: [0,-10,5,null,-3,null,9] is also accepted:

Example 2:

Input: nums = [1,3]

Output: [3,1]

Explanation: [1,3] and [3,1] are both a height-balanced BSTs.

Constraints:

• 1 <= nums.length <= 104
• -104 <= nums[i] <= 104
• nums is sorted in a strictly increasing order.

Solution

from typing import List, Optional

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

class Solution:
    def sortedArrayToBST(self, nums: List[int]) -> Optional[TreeNode]:
        # Set up recursion
        return self._makeTree(nums, 0, len(nums) - 1)

    def _makeTree(self, nums: List[int], left: int, right: int) -> Optional[TreeNode]:
        # left as lowest# can't be greater than right
        if left > right:
            return None
        # Set median# as node
        mid = (left + right) // 2
        mid_node = TreeNode(nums[mid])
        # Set mid node's kids
        mid_node.left = self._makeTree(nums, left, mid - 1)
        mid_node.right = self._makeTree(nums, mid + 1, right)
        # Sends node back || Goes back to prev node
        return mid_node

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