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Problem

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

Code

class Solution {
    public TreeNode sortedArrayToBST(int[] nums) {
        return helper(nums, 0, nums.length - 1);
    }

    private TreeNode helper(int[] nums, int start, int end){
        if(start > end) return null;

        int mid = (start + end) / 2;
        TreeNode root = new TreeNode(nums[mid]);

        root.left = helper(nums, start, mid - 1);
        root.right = helper(nums, mid + 1, end);
        return root;
    }
}