题目：

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

思路：

首先找到数组中间位置的结点，并以此创建一个新的跟结点。向两边递归。注意当开始和结束位置一样时返回null。

代码：

public TreeNode sortedArrayToBST(int[] nums) {
       if(nums == null || nums.length == 0)
           return null;
       return findNode(nums, 0, nums.length);
 }
    
private TreeNode findNode(int[] nums, int begin, int end){
    if(begin == end)
        return null;
    int n = (end + begin)/2;
    TreeNode root = new TreeNode(nums[n]);
    root.left = findNode(nums, begin, n);
    root.right = findNode(nums, n+1, end);
    return root;
}