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

解题思路：根据递增序列，求平衡二叉查找树，根据平衡二叉查找树的性质，其左右都是二叉查找树，用递归可以比较轻松求解。但要注意边界问题及下标的起始位置。

/**

* 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 *getBST(vector<int> &num,int start,int end)

{

if(start>end)return NULL;

int mid = (start + end)>>1;

TreeNode *root = new TreeNode(num[mid]);

root->left = getBST(num,start,mid-1);

root->right = getBST(num,mid+1,end);

return root;

}

TreeNode *sortedArrayToBST(vector<int> &num) {

if(num.size()==0)return NULL;

else return getBST(num,0,num.size()-1);//attention here

}

};