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

基本上一次过，要注意边界条件的问题：如果在recursion里有两个参数int begin, end, 递归写作recursion(num, mid+1, end), 因为+号的原因，递归中是会出现begin > end 的情况的，所以考虑初始条件的时候应该要考虑充分。

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         if (num == null || num.length == 0) return null;13         return construct(num, 0, num.length - 1);14     }15     16     public TreeNode construct(int[] num, int begin, int end) {17         if (begin > end) return null; 19         int mid = (begin + end) / 2;20         TreeNode root = new TreeNode(num[mid]);21         root.left = construct(num, begin, mid-1);22         root.right = construct(num, mid+1, end);23         return root;24     }25 }