Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

[解题思路]

递归确定每棵树的根节点的值，这里根节点的值是二分搜索的中值

由于这里是有序数组，确定root的时间复杂度为O(1), 整个算法的时间复杂度为O(n)，n为节点数目。

1 /** 2  * Definition for binary tree 3  * public class TreeNode { 4  *     int val; 5  *     TreeNode left; 6  *     TreeNode right; 7  *     TreeNode(int x) { val = x; } 8  * } 9  */10 public class Solution {11     public TreeNode sortedArrayToBST(int[] num) {12         // Start typing your Java solution below13         // DO NOT write main() function14         return generate(num, 0, num.length - 1);    15     }16     17     public TreeNode generate(int[] num, int start, int end){18         if(start > end){19             return null;20         }21         int mid = (end + start) / 2;22         TreeNode root = new TreeNode(num[mid]);23         root.left = generate(num, start, mid - 1);24         root.right = generate(num, mid + 1, end);25         return root;26     }27 }