Convert Sorted Array to Binary Search Tree -- Leetcode

Question:
Given an array where elements are sorted in ascending order, convert it to a height balanced BST.
Answer:
/**
 * 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) {
        if(num.size()==0)return NULL;
        int len= num.size();
        TreeNode *root = ArraytoBSTHelper(num, 0, len-1);
        return root;
    }
    TreeNode *ArraytoBSTHelper(vector<int> &num, int l, int h){
        if(l>h) return NULL;
        int mid = (l+h)/2;
        TreeNode *root = new TreeNode(num[mid]);
        root->left = ArraytoBSTHelper(num,l,mid-1);
        root->right = ArraytoBSTHelper(num,mid+1,h);
        return root;
    }
};