Leetcode: Convert Sorted Array to Binary Search Tree

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 }