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

Solution

Start in the middle and add the other nodes recursively to left and right (depending on what we read from the middle). Stop when left starts to be bigger than right (length of the array minus one).

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode sortedArrayToBST(int[] nums) {
        return createTree(nums, 0, nums.length - 1);
    }

    private TreeNode createTree(int[] nums, int left, int right) {
        if (left > right) return null;

        int mid = (left + right + 1) / 2;
        TreeNode node = new TreeNode(nums[mid]);

        node.left = createTree(nums, left, mid - 1);
        node.right = createTree(nums, mid + 1, right);

        return node;
    }
}