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

balanced tree 是指 tree的 min depth 和max depth不超过某个数值（1）。而不是完全二叉树!!!!

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         // IMPORTANT: Please reset any member data you declared, as13         // the same Solution instance will be reused for each test case.14         return buildTree(num, 0, num.length - 1);15     }16     public TreeNode buildTree(int[] num, int s, int e){17         if(s > e) return null;18         int mid = (s + e) / 2;19         TreeNode root = new TreeNode(num[mid]);20         root.left = buildTree(num, s, mid - 1);21         root.right = buildTree(num, mid + 1, e);22         return root;23     }24 }