Convert Sorted Array to Binary Search Tree

来源：Leetcode

原帖：https://oj.leetcode.com/problems/convert-sorted-array-to-binary-search-tree/

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

代码：

 /**  
  * Definition for binary tree  
  * struct TreeNode {  
  *   int val;  
  *   TreeNode *left;  
  *   TreeNode *right;  
  *   TreeNode(int x) : val(x), left(NULL), right(NULL) {}  
  * };  
  */  
 class Solution {  
 public:  
   TreeNode *sortedArrayToBST(vector<int> &num) {  
     return buildBST(num, 0, num.size() - 1);  
   }  
     
   TreeNode *buildBST(vector<int> &num, int start, int end) {  
     if (start > end) return NULL;  
     int mid = start + (end - start) / 2;  
     TreeNode *root = new TreeNode(num[mid]);  
     root->left = buildBST(num, start, mid - 1);  
     root->right = buildBST(num, mid + 1, end);  
     return root;  
   }  
 };