Convert Sorted Array to Binary Search Tree

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, as

13         // 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 }