Convert Sorted Array to Binary Search Tree

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

Code:

/**
* 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 *res;
TreeNode *BST(vector<int> &num, int begin, int end){
if(begin>end)
return NULL;
int mid=(begin+end)/2;
TreeNode *node = new TreeNode(num[mid]);
node->left=BST(num,begin,mid-1);
node->right=BST(num,mid+1,end);
return node;
}
TreeNode *sortedArrayToBST(vector<int> &num) {
return BST(num,0,num.size()-1);
}
};