[LC] 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.

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

/**
 * 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) {
        if (nums == null || nums.length == 0) {
            return null;
        }
        return buildBST(0, nums.length - 1, nums);
    }

    private TreeNode buildBST(int start, int end, int[] nums) {
        if (start > end) {
            return null;
        }
        int mid = start + (end - start) / 2;
        TreeNode cur = new TreeNode(nums[mid]);
        cur.left = buildBST(start, mid - 1, nums);
        cur.right = buildBST(mid + 1, end, nums);
        return cur;
    }
}