Convert Sorted Array to Binary Search Tree

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

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