leetcode------Convert Sorted Array to Binary Search Tree

标题：	Convert Sorted Array to Binary Search Tree
通过率：	33.8%
难度：	中等

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

给定的数组是递增的，那么平衡的二叉树一定是从中间开始，左树递归，右树右递归。看代码

 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         return buildTree(num,0,num.length-1);
13     }
14     public TreeNode buildTree(int[] num,int left,int right){
15         if(left>right)return null;
16         int mid=(left+right)/2;
17         TreeNode root=new TreeNode(num[mid]);
18         root.left=buildTree(num,left,mid-1);
19         root.right=buildTree(num,mid+1,right);
20         return root;
21     }
22 }