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);
    }
};